Looks like RockStar’s latest patch has them running scared from modders. In an attempt to keep online play safe and fair, R* has patched out modding. As I’m sure you’re disappointed about that, one youtuber has already reimplemented the script hook required for modding.

Taligolt here has a guide on exactly how to re-mod your game yourself. This may unfortunately have the side effect of re-enabling mods online as well, something that’s plagued public games since only a few days after release.