If you get an alert about the SIM card in your iPhone - Apple Support - https://support.apple.com/HT201420 - If you get an alert that says Invalid SIM or No SIM Card installed, follow these steps.
If you can't activate your iPhone - Apple Support - https://support.apple.com/HT201407 - "If your iPhone can't be activated or an alert says the activation server is unavailable [or cannot be reached] or the SIM card is unsupported, learn what to do."