GMC Troubleshooting Questions? Ask a Mechanic for Answers
From the fact that you said you are jump starting the vehicle and code C0899 refers to the battery voltage less than 9 volts, it appears there may be a problem with the charging system.
Code U1064 refers to a Lost Communications With Body Control Module (BCM).
If the generator battery control module loses communication with the body control module (BCM), the module will set U1064.
For more info on those codes, click here.
I would have the charging system checked. Usually any auto-parts store will check it for free.