Comment 8 for bug 1885357

That bug mentions:

    This patch allows the capability, but checks validity at the
    API layer before issuing the create call. There are two
    requirements for the new type specification to be valid:
    1. There is only one backend (cinder-volume) topic configured
    2. Both types in question specify the same volume_backend_name
    If neither of these requirements are met, the user will receive
    an "invalid type" error explaining that the type combination is
    not compatible and that they should omit the type argument altogether.

    Change-Id: I08bc5e9a8800ce3b27c7db90b7bff86d7d14359a
    Closes-Bug: #1289931

In my environment, we have 6 enabled_backends (2 AZs with 3 backends in each one).

@Akihiro: Do you have just one enabled_backends in your cinder.conf?