I had the same error plugin class not found: spark.sql.catalog... is not defined immediately after attaching the workspace into Unity catalog.

The error was resolved by restarting SQL Warehouse.

It seems that if SQL Warehouse (or any cluster) is running at the time the workspace is attached into Unity catalog, then it shows this error.

Maybe it is a bug as the Unity catalog was visible in the Catalog menu directly, but all operations on top of this catalog were not working until the compute restart?