SQLCipher - cant decrypt database

Im trying to decrypt a database with SQLCipher. Looking for help, i found this link but i have a problem while attaching the database.

So, when i try to perform ATTACH DATABASE 'test.db' AS plaintext KEY ''; im getting the following error: Error: file is encrypted or is not a database

Ive just downloaded latest version of SQLCipher. I thought that it could be a problem with the SQLite version, so i also downloaded the latest version of SQLite.

What am i doing wrong?

Thanks and regards.