Naqi

Those are all updated as you can see the dates of logs,

I have included all fatal logs in last response

This is the only occurrence in my theme , I have found the function in this file wp-content/plugins/wc-multivendor-membership/core/class-wcfmvm.php

on these lines
wp-content/plugins/wc-multivendor-membership/core/class-wcfmvm.php:1278: $order_id = $subscription->get_parent();
wp-content/plugins/wc-multivendor-membership/core/class-wcfmvm.php:1347: $order_id = $subscription->get_parent();
wp-content/plugins/woocommerce/includes/legacy/abstract-wc-legacy-product.php-bak:416: public function get_parent() {
wp-content/plugins/woocommerce/includes/legacy/abstract-wc-legacy-product.php:416: public function get_parent() {
wp-content/plugins/woocommerce-product-addons/classes/class-product-addon-display.php:458: if ( $product && $product->get_parent() && is_single( $product->get_parent() ) && $this->check_required_addons( $product->id ) ) {

here you the Fatal Errors

2019-05-01T16:19:00+00:00 CRITICAL Uncaught Error: Call to undefined function rocket_purge_cloudflare() in /home/pnubdgksau/public_html/wp-content/plugins/wp-rocket-1/inc/common/purge.php:429
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): do_admin_post_rocket_purge_cloudflare(”)
#1 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)
#2 /home/pnubdgksau/public_html/wp-includes/plugin.php(465): WP_Hook->do_action(Array)
#3 /home/pnubdgksau/public_html/wp-admin/admin-post.php(70): do_action(‘admin_post_rock…’)
#4 {main}
thrown in /home/pnubdgksau/public_html/wp-content/plugins/wp-rocket-1/inc/common/purge.php on line 429

2019-05-01T16:20:16+00:00 CRITICAL Uncaught Error: Call to undefined function rocket_purge_cloudflare() in /home/pnubdgksau/public_html/wp-content/plugins/wp-rocket-1/inc/common/purge.php:429
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): do_admin_post_rocket_purge_cloudflare(”)
#1 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)
#2 /home/pnubdgksau/public_html/wp-includes/plugin.php(465): WP_Hook->do_action(Array)
#3 /home/pnubdgksau/public_html/wp-admin/admin-post.php(70): do_action(‘admin_post_rock…’)
#4 {main}
thrown in /home/pnubdgksau/public_html/wp-content/plugins/wp-rocket-1/inc/common/purge.php on line 429
2019-05-05T16:41:49+00:00 CRITICAL Uncaught Error: Call to undefined method WC_Product_Simple::get_available_variations() in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php:226
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): dcwd_populate_ninja_forms_select_for_variations(Array, Array)
#1 /home/pnubdgksau/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#2 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Render.php(253): apply_filters(‘ninja_forms_ren…’, Array, Array)
#3 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/ninja-forms.php(737): NF_Display_Render::localize(3)
#4 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Shortcodes.php(27): Ninja_Forms->display(‘3’)
#5 /home/pnubdgksau/public_html/wp-includes/short in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php on line 226

2019-05-05T16:51:45+00:00 CRITICAL Uncaught Error: Call to undefined method WC_Product_Simple::get_available_variations() in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php:36
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): dcwd_populate_ninja_forms_select_for_variations(Array, Array)
#1 /home/pnubdgksau/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#2 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Render.php(253): apply_filters(‘ninja_forms_ren…’, Array, Array)
#3 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/ninja-forms.php(737): NF_Display_Render::localize(3)
#4 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Shortcodes.php(27): Ninja_Forms->display(‘3’)
#5 /home/pnubdgksau/public_html/wp-includes/shortc in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php on line 36
2019-05-11T11:35:50+00:00 CRITICAL Uncaught Exception: DateTime::__construct(): Failed to parse time string (DD-MM-YYYY) at position 0 (D): The timezone could not be found in the database in /home/pnubdgksau/public_html/wp-content/plugins/martfury-addons/inc/shortcodes.php:3857
Stack trace:
#0 /home/pnubdgksau/public_html/wp-content/plugins/martfury-addons/inc/shortcodes.php(3857): DateTime->__construct(‘DD-MM-YYYY’)
#1 /home/pnubdgksau/public_html/wp-includes/shortcodes.php(325): Martfury_Shortcodes->countdown(Array, ”, ‘martfury_countd…’)
#2 [internal function]: do_shortcode_tag(Array)
#3 /home/pnubdgksau/public_html/wp-includes/shortcodes.php(199): preg_replace_callback(‘/\\[(\\[?)(martfu…’, ‘do_shortcode_ta…’, ‘[martfury_count…’)
#4 /home/pnubdgksau/public_html/wp-content/plugins/js_composer/include/classes/editors/class-vc-frontend-editor.php(686): do_shortcode(‘[martfury_count…’)
#5 /home/239991.cloudway in /home/pnubdgksau/public_html/wp-content/plugins/martfury-addons/inc/shortcodes.php on line 3857

here is the Fatal error logs
WooCommerce -> Status -> Logs -> fatal_errors log -> is any error generated?

2019-05-05T16:41:49+00:00 CRITICAL Uncaught Error: Call to undefined method WC_Product_Simple::get_available_variations() in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php:226
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): dcwd_populate_ninja_forms_select_for_variations(Array, Array)
#1 /home

/pnubdgksau/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#2 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Render.php(253): apply_filters(‘ninja_forms_ren…’, Array, Array)
#3 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/ninja-forms.php(737): NF_Display_Render::localize(3)
#4 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Shortcodes.php(27): Ninja_Forms->display(‘3’)
#5 /home/pnubdgksau/public_html/wp-includes/short in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php on line 226

2019-05-05T16:51:45+00:00 CRITICAL Uncaught Error: Call to undefined method WC_Product_Simple::get_available_variations() in /homepnubdgksau/public_html/wp-content/themes/martfury-child/functions.php:36
Stack trace:
#0 /home/pnubdgksau/public_html/wp-includes/class-wp-hook.php(286): dcwd_populate_ninja_forms_select_for_variations(Array, Array)
#1 /home/pnubdgksau/public_html/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)
#2 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Render.php(253): apply_filters(‘ninja_forms_ren…’, Array, Array)
#3 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/ninja-forms.php(737): NF_Display_Render::localize(3)
#4 /home/pnubdgksau/public_html/wp-content/plugins/ninja-forms/includes/Display/Shortcodes.php(27): Ninja_Forms->display(‘3’)
#5 /home/pnubdgksau/public_html/wp-includes/shortc in /home/pnubdgksau/public_html/wp-content/themes/martfury-child/functions.php on line 36`

I was also getting same errors.
I increased PHP memory as described above.
after that
I just renamed plugins folder to plugins.bk
Updated WordPress.
renamed plugins
Updated All plugins.
Solved.