Cannot shift when engine running, and mower keeps trying to move forward.

Installed new belt. Ran following test:
Park Brake ON
Wheels chocked
Tranny in neutral
Clutch Depressed or Not depressed (makes no difference)

Transaxle pulley still turns.

Conclusion: clutch stuck in engaged mode.

Briggs & Stratton, vertical, gear tranny (Peerless 801F)

How does one Unstick or disengage a clutch?

Engine turned off, depressed clutch pedal, no movement what so ever, even though the
Bar is pressing on top of the clutch mechanism.

Guidance please, point me in the right direction to start to fix this.
TIA

Was this the drive belt you replaced

Yes, however, the clutch was doing the same thing prior to replacing the drive belt.
I verified the tranny movement by marking the pulley with white paint and using a mirror to reflect from under the rear cover, while I was sitting on the seat. Only difference between having clutch pedal released or depressed is: A loud HUM sound occurs when the clutch pedal is depressed and stops when I release the clutch pedal.