Mortise Lock is Locked and Stuck
Thanks for posting. Can you try manually throwing the mortise with a slotted screwdriver? Remove the key lock and then stick the screwdriver in the slot where the tail of the key lock usually goes. Whether or not you can unlock the mortise this way will tell us whether or not there was simply wrong with your handle set installation, or the mortise itself.
The part that sticks in the locked position is on the bottom if installed correctly.
Try putting the handle in the unlocked (hopefully down) position. Then lift the lead edge of the door as much as possible and pull open.
Good luck.
If the screw driver cannot get the lock to budge then most likely the mortise lock is broken and needs to be replaced. Since your handle set is the 82-081, either the 82-238 or 82-268B should be compatible. Look them over and see what dimensions make most sense.
In addition, our friend from California has some ideas you might want to look over!