I was a bit skeptical such message would be displayed, but browsing thru the service manual and owners manual, found this info:
REPLACE LIFTGATE FUSE
This message will be displayed if the liftgate lock malfunctions and/or the liftgate fuse blows. For more information on fuses, see Fuses and Circuit Breakers . If the message returns after replacing the fuse, see your GM dealer for service.
I would check first to make sure the lock in the rear gate works. All other doors lock ok? Have a look at the rear liftgate lock relay. Also check for grounds if they are not corroded, loose. My guess would be that the lock assembly is stiff, its a common issue on those trucks. I didnt know it would cause an error message tho. Liftgate lock fuse is in the dashboard panel ECC fuse 10 amps. I included a few pics-diagrams from the service manual. Keep us posted.