Problems adding a custom field for products in WooCommerce


Basically I'm trying to follow what is suggested in the below link:

I'm trying to add a custom field called "custom_permalink" when editing a product in WooCommerce.

The data is added and a yellow frame is flashing to acknowledge. But when I click the update button for the product the custom field is not added.

I read in the below post that it can be a problem with functions.php. But I never edited that file. Can you please advice?

I added the field custom_permalink through the plugin called: Custom Permalink

This plugin adds "custom_permalink" to the Custom fields dropdown menu.

When I add a value to custom_permalink and click the "Add custom field" button it flashes yellow. But when I click the Update button to update the product the value is cleared and not saved.

What can be wrong?