This is what causes shift inhibit
The following conditions may cause an inhibited state:
Engine speed too high
Throttle percentage incorrect
Output speed movement
The transmission control system inhibits shifting to protect the transmission
from some types of abusive operation, in response to Diagnostic Trouble
Codes (DTCs), and to satisfy transmission feature/option requirements.
These shift inhibits fall within the following types:
Above idle neutral to range shifts
Forward/reverse directional shifts
Transmission problems
Auxiliary equipment operation