Here are some solutions that have worked for other users:
Go to Documents>Electronic Arts>TheSims4, find the GameVersion file. Open it, and see what number is written. Check to make sure it's the same number as your game version (PC: 1.72.28.1030 or Mac: 1.72.28.1230) if it isn't, change the number to your version in the GameVersion file and save.
Make sure that your antivirus is not blocking your changes: https://answers.ea.com/t5/Technical-Issues-PC/CURRENT-ISSUE-Anti-Virus-blocking-User-Folder-Screenshots/td-p/9572695
Go to Documents>Electronic Arts>TheSims4 and delete the options.ini file (this will reset your game settings).
Try a game repair (origin>click on the sims4 in your library and choose repair).
If nothing works you can post your problems on Answer HQ, here is the original thread: https://answers.ea.com/t5/Technical-Issues-PC/Can-t-activate-my-mods-after-new-update/td-p/9560277