I had the same problem, so I used testing cheats and shift clicked my sim > public image > increased my Sims fame until I was equal to the celebrity in which I wanted to interact with.
After that I began to interact with that celebrity completely normal like I would any other sim.
After one conversation the celebrity should show up as an acquaintance in your sim’s relationship box!
You are now safe to decrease your celebrity status via testing cheats, shift click > public image > decrease fame back to whatever level you want to be..
However you can only decrease two levels so if you need to decrease more than that just go into your Sims star info panel and quit the spotlight! This will take you back to zero fame. After that if you need to increase fame just shift click and increase fame again. But don’t forget to enter back into the spotlight via your sims star panel!!
Now you should be right where you started but now you can interact with the celebrity. ☺️
