No Park...bad pawl..or Linkage issue

There are a couple of reasons this can happen.
The knob on the end of the park rod came off (seen it more than once) or the pawl end broke off, or the linkage broke where the park rod goes into it inside the trans.
If everything else seems normal, but park won't lock in you might get away with just pulling the tailshaft housing and replace the pawl or knob on the end of the park rod.

Welding or brazing it back on works just fine for the rod end but you would be best off to relace the pawl if it broke.

Here is the rod that sticks out into the tail housing and the end might have come off, and the pin that hold the pawl on the case.
Both accessible with the tail housing removed.