U0121 Chevrolet - just had a transmission rebuild, could anything be related to this?

Question

Brake lights ABS light traction control and engine lights all come on

Answer 1

U0121 Lost communication with electronic brake control.

With this type of code you need to gather any DTC code that are stored, depending on the code that you get from the transmission control module, and the engine control module. U0120 can set by a bad ground located on the under frame, check that ground for proper connection