I know there's been other posts and troubleshooting about defective servo motors, and I ran into the same issue. The servo motor that I ordered on Amazon did not work.
The first servo motor I ordered was from the Treedix brand off of Amazon. It came as a 2 pack, and both servos were defective.
I then ordered Deegoo-FPV servo motor which came with 4 servos from Amazon. The first one I tried WORKED.