Theres a large-ish tab on the other side of the connector that is in the center of the picture. Press that down and give it a good amount of force. If you have someone to assist or you dont mind wedging the taillight assembly on the tailgate, you could remove the taillight and guard and pull the harness out until the connector is outside of the body, it will be much easier that way.
EDIT: the closer I look at the photo, I think the second connector on the left side of the picture might be a quick splice connector for a third brake light harness, such as a 4-pin one for a trailer. I had one of those which had to be cut off to remove the taillights. If that is a 3rd light harness, and it is quick-connected to the taillight pigtail, it will have to be cut off to fully remove the tail light. If it is instead spliced into the wiring harness that the taillight connects to, you will be fine.