This product is currently out of stock and unavailable issue

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I get this error:

This product is currently out of stock and unavailable.

in some of my files..

This is product where that issue is showed..

This issue makes my products unavailable to purchase.. I searched and found this code:

// Increase WooCommerce Variation Limit
function custom_wc_ajax_variation_threshold( $qty, $product ) {
return 100;
}

add_filter( 'woocommerce_ajax_variation_threshold',
'custom_wc_ajax_variation_threshold', 100, 2 );

that saying to me to insert into functions.php file in theme, but i dont see any changes from it.