From the Pelican Youtube video (
https://youtu.be/ayBiABwGpo4), it looks to me like you need to insert the small flat-head screwdriver into the small square hole below where the red arrow is pointing. (I'm assuming you're watching the same video I've linked to, above.)
I haven't taken off the tail lights in my 997.2 C2S, but a lot of the electrical connectors function this way -- the square hole captures a catch. Pushing it in allows the connector to be separated. Other connectors don't have this hole, and you have to slide in a thin-bladed screwdriver to release the catch, allowing the connector to separate.