The manual says to remove the cap on the coolant reservoir when the engine is cold. Add coolant until the level is at the Min line on the pressurized side of the tank. With the cap still removed start the engine and wait until the temp reaches 196 deg F.
Then slowly keep adding coolant. Once the first tank (the pressurized side) reaches the full/min line it will spill over into the non-pressuried side (second tank). Keep adding coolant untill the second tank is 1/3 full. Replaced the cap. Then check the level in the tanks in a day or so when the engine is cold to see if it changed.
If the coolant level is still low after having followed these steps twice, have the coolant system checked by a certified technician at the dealer for a possible leak.
Page 10-22 https://my.chevrolet.com/content/dam...sel-Manual.pdf