Fatal error when WooCommerce inactive

Fatal error: Uncaught Error: Undefined constant "WC_VERSION" in /path/to/wp-content/plugins/pricing-deals-for-woocommerce/woo-integration/vtprd-parent-functions.php:5434 
Stack trace: 
#0 /path/to/wp-content/plugins/pricing-deals-for-woocommerce/vt-pricing-deals.php(171): require_once() 
#1 /path/to/wp-includes/class-wp-hook.php(303): VTPRD_Controller->vtprd_controller_init('') 
#2 /path/to/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters(NULL, Array) 
#3 /path/to/wp-includes/plugin.php(470): WP_Hook->do_action(Array) 
#4 /path/to/wp-settings.php(578): do_action('init') 
#5 /path/to/wp-config.php(79): require_once('/Users/brianhen...') 
#6 /path/to/wp-load.php(50): require_once('/Users/brianhen...') 
#7 /path/to/wp-admin/admin.php(34): require_once('/Users/brianhen...') 
#8 /path/to/wp-admin/plugins.php(10): require_once('/Users/brianhen...') 
#9 {main} thrown in /path/to/wp-content/plugins/pricing-deals-for-woocommerce/woo-integration/vtprd-parent-functions.php on line 5434

if ( version_compare( WC_VERSION, '3.1.2', '>=' ) ) {   
  add_action('woocommerce_new_product_variation',   'vtprd_update_session_ts_on_price_change') ;
} else {
  add_action('woocommerce_create_product_variation',   'vtprd_update_session_ts_on_price_change') ;  
}

which you could change to

if ( defined( 'WC_VERSION' ) && version_compare( WC_VERSION, '3.1.2', '>=' ) ) {

but there’s still other places that need that check too.

That code is running on init, so another option is to move those add_actions into functions called on woocommerce_init or woocommerce_loaded.