Is this a new group or a group that already has VCs?
Check the group template version -> should be showing in the top right of the Instant Config frame:
The template version should match the deployed firmware version. Depending on the template version - earlier Firmware versions did not allow TACACS.
If this is a new group w/o devices, you can just update the template from Instant Config -> AirWave -> AirWave Settings -> Policy version #.#.# migrate to... select newer version, -> Save (at top right)