How to fix sim error

Hi, first thing to try is taking out the SIM card and put it back in , try not to touch the metal contacts on the back. Also try restarting your iPhone , check Apple’s support page Force restart iPhone – Apple Support (UK). If this does not work you may need to contact your service provider to see if they can provided you with another sim.

if it was me then I would test the sim card in another phone to verify it's in working order, if not then I would contact my carrier about getting a replacement, if it did work then I would assume the iphone sim slot being defect and

iPhone Repair & Service - Apple Support