Activation Error
Some people reported that changing the system date by a few months, launching the game, quitting the game, changing the date back, and relaunching the game fixed this issue for them.
https://steamcommunity.com/app/271590/discussions/0/405694115210006486/ https://support.rockstargames.com/hc/en-us/community/posts/203639797-Activated-GTA-V-PC-on-RSC-website-but-launcher-wants-me-to-activate-again-