As you have stated, you are now the NEW OWNER,
and you are in the dark what the PO has worked on,
TRIED to work on, and or Screwed UP.
There is also a 10MM head bolt on the SIDE of the transmission,
not to be confused with the Shift Lever alignment bolt.
This holds a spring loaded "Stop" for the reverse shift rod inside the tranny.
If this is out/gone or was taken out and the shifter moved, this "Stop" can or will
flip over, keeping the reverse gear engaged, in so locking up the transmission.
Bad News Bears if this happened, tranny has to come out and the Rear section has to be remove,
and maybe even open the front half to get the gears all lined up again, the bolt has to be re-installed AFTER the tail/back housing is put on with all gears in neutra for the rail hold STOP.
If you have to do this, send me a PM and I can save you a lot of headaches and time on Tranny
removal and help walk you thru fixing the Reverse "Stop" fix.
PS You have a Shop Crane/ Cherry Picker/engine hoist?