Here is the code:
B2121 Starter Output Open/BCM Malfunction
Remove, clean and check the connection at the starter from the battery (positive). Remove, clean and resecure the battery cables at the battery terminals (positive off first, negative on last). If that does not solve it, make sure the transmission is in neutral, pull in the clutch and try jumping from the positive battery terminal to the starter solenoid terminal. If you have someone to help you it will be easier. See if that makes the starter turn over. If so, then it is in the switch or the starter relay. If not, then it is probably the BCM.
Cheers,
TQ