The error means that SQL cannot open the file. It doesn't mean anything else.

Double-check there's not a spelling mistake somewhere. Double check that SQL has access to the files (your account is completely and totally irrelevant). If it's a cluster, check the drive dependencies. Check that the folder is not compressed (ntfs compression), though that usually gives different errors. Check that the files are read-write.

Post the attach statement, post any messages in the SQL error log after the attach.