In Harness Feature Flags, you can add a target rule to disable an enabled flag, but not to enable a disabled flag: target variations are only evaluated when the flag is ON. It looks like a bug but it is how the evaluator works.
Flip the flag ON and make the default rule serve False, then add target rules that serve True for the users you want. That gives you off-for-everyone except specific targets, which is the intended way to do it.