You should troubleshoot the problem with your carrier.
You can take it out and place it back in a few times. Also, wipe it with an alcohol wipe (the one for cleaning glasses), and place it back in. If that doesn't work, you could have a failed SIM or a failing SIM slot inside the phone.
Do you have a SIM card? If so, restart. If that doesn't work, you may need to take in?
No just go buy a sim card
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