I just bought a iPhone X from someone and it’s saying invalid sim, how can I fix this?

Sometimes the carrier will unlock for you, sometimes only for the holder of the previous contract, and there’ve been cases were some folks have ended up “inheriting” the remainder of the previous owner’s cell plan.

How to unlock your iPhone for use with a different carrier - Apple Support

You bought a carrier locked device and only the carrier can remove it.

What is the exact error message that you got? Is the phone carrier locked?

If you know who the carrier is, they will tell if you qualify to get it unlocked.