In Microsoft SQL Server 2008 when attaching a database, you may get an error saying “Database cannot be opened because it is version 661. This server supports version 655 and earlier.
This means you are trying to restore SQL 2008 R2 database. This database format is not backwards compatible, so you will have to upgrade to 2008 R2 or convert the database to 2008 format on 2008 R2 instance,
Download Microsoft SQL Server 2008 R2 Express for free at Microsoft.