You cannot turn on BitLocker auto-unlock feature for a BitLocker protected data drive although the drive is really BitLocker protected. You are actually able to manually unlock the protected data drive. But when you try to turn on auto-unlock, the error message displayed is:
BitLocker Drive Encryption
The operating system drive is not protected by BitLocker Drive Encryption
The error message is somewhat misleading. Though it does not mention your data drive which you want to enable the auto-unlock for. It actually tells you that your operating system drive (different partition on a different disk probably) is not yet protected by BitLocker Drive Encryption.
You cannot turn auto-unlock feature on unless the operating system drive is also encrypted with BitLocker as well.