Can't Make Alien Sim in CAS

You could try repairing the game or uninstalling and reinstalling the game. 

I'm also having this issue and have tried repairing the game?

Try the following

• Close the game
• Rename Documents\Electronic Arts\The Sims 4 to Documents\Electronic Arts\The Sims 4_Old
• Repair the game
• Start the game. All your content will be gone now as the game created a new folder. All your saves/.. is still in the old folder so not to worry. 
• Start a new save game
• Check if you still have the error
• Report back with your results for further steps on how to get your stuff back.

After creating a new game file as advised above I removed a corrupt file that was causing the issue for me its called  “Eyes.package” its an old CC that needs updating.

Hope this helps.

I did this but I can't see any of my cc or saves, I've restarted my game and nothing is working :(

I’m having the same issue right now when I try to make an alien sim it keeps loading and won’t work I have deleted an eye presets for alien and it still doesn’t work for me and I also repaired the game and still doesn’t work .

Did you try a folder reset ?