Microsoft Purview - Free to Enterprise Account Not Working

You should be able to upgrade the Purview account to Enterprise in the account setting page, however if there are already existing Purview accounts then there maybe additional steps to pick one to become the default or if you do not have one you will have to provide Azure subscription information so an account can be created.  https://learn.microsoft.com/en-us/purview/upgrade#upgrade If you are still not able to upgrade please open a support ticket.