My SIM card is not showing what do I do

Hello first I would suggest you taking out the sim and putting it back in and also have you tried to put it in another device ?. does it have the same issue ?, if so please contact your carrier so they can send you a replacement sim card. if this didn't help then you can try contacting apple support or by going back to the company or place where you bought the phone any they can help you with that issue and get it solved.