How to fix fatal error?

Fatal error: Uncaught Error: Call to undefined function gutenberg_can_edit_post_type() in /home2/viedesi1/public_html/wp-content/themes/thegem/functions.php:2645 Stack trace: #0 /home2/viedesi1/public_html/wp-includes/class-wp-hook.php(286): thegem_gutenberg_replace_default_add_new_button(”) #1 /home2/viedesi1/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array) #2 /home2/viedesi1/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #3 /home2/viedesi1/public_html/wp-admin/admin-header.php(118): do_action(‘admin_print_scr…’) #4 /home2/viedesi1/public_html/wp-admin/edit.php(303): require_once(‘/home2/viedesi1…’) #5 {main} thrown in /home2/viedesi1/public_html/wp-content/themes/thegem/functions.php on line 2645

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