There are multiple reasons why the Exhaust By Pass Valve can be inoperative.

It is operated by a complicated vacuum system. A quick test of the actuator is to disconnect the small pipe on the actuator, attach a suitable pipe that you can suck on
and the actuator should move and open the butterfly.

In my case on F3 YEL, the lever had become detached from the butterfly spindel.
It was an easy fix while the engine was out for a service, it just needed a touch of weld.

I know some cars have the exhaust bypass wired permanently open, however I understand it can cause the catalytic converters to overheat.