Insery SIM appears on Nokia mobile phone. What should I do?

see if you have bad contact. Try a few more times! There may also be a system problem (but it is generally unlikely). If it is really useless, upgrade the software on the official website, or brush the machine 9. It may be that the contact between the SIM card and the card slot is not in good contact! If it really doesn't work, just ask the mobile phone repairer.