Error 121 is a generic “failed to program” message, so this can have several causes.
Tech Tool 2.8.130
Solution 1:
One common cause is low battery Voltage to the ECU that is being programmed.
Solution 2:
Check all connections, and ensure that the vehicles batteries are good and
fully charged. You can also remove power from the ECU that is being programmed, wait,
connect, and try a second time.
Solution 3:
If you still get error 121, remove power for a short time, try again with a different adapter.
Vocom , NexIQ, Noregon DLA or Dearborn.