My iphone7 can't detect my sim card

If you get an alert about the SIM card in your iPhone - Apple Support

Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo

If that does not work, make an appointment at the Genius Bar to have the phone evaluated unless you are able to check the sim card in an another phone and determine the sim card is the problem. If it is the sim card, get a new one from your carrier.

Try restarting your phone. First Power down your phone, then take out your sim card. Keep phone off for a good 3-5 minutes. put sim card back in phone before you power up your device