When the server is started up with enableAccessToUserRoles=false, any queries using the $$USER_ROLES builtin variable will fail. The error message today simply states:

Builtin variable $$USER_ROLES is not available

This is the default error message reported when any variable is used in an aggregation.

We could update that error message to say something like

Builtin variable $$USER_ROLES is not available since the server is not configured to accept it

This would provide some extra context as to why $$USER_ROLES is unavailable.