SQL Server 2008 databases are version 655 whereas SQL Server 2008 R2 databases are 661.
You are trying to attach a 2008 R2 database (v661) to a 2008 instance which is not supported. Once the database has been upgraded to an 2008 R2 version, it cannot be downgraded.
You'll have to either upgrade your 2008 SP2 instance to R2
you have to copy out the data in that database into an 2008 database
(e.g. using the data migration wizard, or something equivalent).
Look at similar issues discussed here:
DB cannot be opened because it is version 655. This server supports version 612
Version 661. This Server supports version 655
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