Unable to activate a form template even as a site collection administrator

Administrator approved InfoPath forms templates are managed in central administration website and can be activated on each site collections.

To do this, SharePoint requires that administrator has full permissions on the site collection. This is often the case when we are site collection administrators, but some permission can be removed at web application level, which prevents anyone to activate forms templates (including the setup account)

To solve this, go to Central Administration > Application Management > User Permissions for Web Application, select the web application on which you try to activate your form template and check ALL the boxes.

After validating, you should be able to successfully activate your form template on your site collection.