If you have a 2015 GMC Sierra and it says service trailer brake system, there are a few things that could be going on. First, check to see if the trailer is properly connected. If the wiring harness isn't plugged in all the way or is loose, that can trigger this error message.
Next, check the fuse for the trailer brake system. If it's blown, that will also cause this error message. Finally, if neither of those are the problem, it's possible that there is an issue with the actual trailer brakes themselves.
If you're not sure how to fix any of these issues, take your GMC Sierra to a qualified mechanic or dealership service department for assistance.