You can set the requirement that the privilege level attribute (priv-lvl) must be specified for TACACS authentication to occur. Setting the privilege level attribute as required does not change any behavior associated with values received in the priv-lvl attribute, only the presence/absence of the attribute.
Summit X450-G2, X460-G2, X670-G2, X770, and ExtremeSwitching X440-G2, X870, X620, X690 series switches.
configure tacacs priv-lvl [required | optional]