DTC C1214 is a fault in the ABS relay. The relay is built into the electronic brake and traction control module (EBTCM) and can't be serviced in the field.
That the DTC keeps resetting indicates it's a hard code and not an intermittent caused by a wiring/connections problem. Solving the code requires either replacement of the EBTCM or both the EBTCM and the brake pressure modulator (BPMV)
Solving this code requires a scan tester which can command the ABS autobleed function.
Do you have a tester which supports autobleed?
If you do, post to that effect and I can give you more instructions.
If you don't, take the car to a service facility familiar with GM ABS service for repair.