This content database has a schema version which is not supported in this farm

we usually face this issue when we try to mount content db.

It is because of the database version conflict.

Just go to your local database and open Versions table and check the version of your database.

now open the database which you are trying to mount and open the Versions table and change its version as per your database version.

now try to mount your database again.

Mount Content DB:

Mount-SPContentDatabase -Name WSS_Content_DB_ToAttach 

-WebApplicationhttp://mywebapplication:port

Dismount Content DB:

Dismount-SPContentDatabase -Identity WSS_Content_DB_ToAttach

Remove Content DB:

Remove-SPContentDatabase -Identity WSS_Content_DB_ToAttach