Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
You can’t use the FAQ schema for your product in the Rank Math Free version. The FAQ Schema on WooCommerce is only available for the Rank Math PRO version. The FAQ block however is available in the Rank Math Free version.
As for the schema tab in your taxonomy pages, please ensure that the option “Remove Snippet Data” is currently disabled under Rank Math > Titles & Meta > Post: Categories:
Then, head back to your category page editor and you should be able to add a schema:
If you need further assistance, please don’t hesitate to contact us.
If Remove Snippet Data is disabled for the custom taxonomy in the Titles & Meta settings, assuming you are using the latest version of all the plugins and the themes (please update if not yet), it would seem like a conflict with one of the plugins or the theme you are using.