Problem: You are trying to mount a SharePoint content Database but got the error message "This content database has a schema version which is not supported in this farm." as seen below:
Mount-SPContentDatabase : This content database has a schema version which is not supported in this farm.
At line:1 char:1
+ Mount-SPContentDatabase "SharePoint_Farm_Content" -DatabaseServer ...
+ CategoryInfo : InvalidData: (Microsoft.Share...ContentDatabase:
SPCmdletMountContentDatabase) [Mount-SPContentDatabase], SPUpgradeException
+ FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletMountContentDatabase
Solution: The issue often occurs when you are trying to attach a content database to a SharePoint Server which has a different version level as the current one. For example the former content database was attached to a SharePoint server which was running SharePoint 2010 with SP1. But the current new server has only a fresh 2010 installation without SP1. To solve that upgrade your SharePoint 2010 Server as mentioned for example here.