No sim available

1.

Try this -> remove and reinsert the SIM card.

If it doesn't help visit your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Ask also to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

2.

See also these Apple articles below:

If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

Some resources about a badly fitting SIM card and one about exactly the issue you describe:

This document will help you determine what size SIM your device takes:

Learn which size SIM card your iPhone or iPad uses - Apple Support

Learn how to remove or transfer the SIM card in your iPhone or iPad (Wi-Fi + Cellular models).

Remove or switch the SIM card in your iPhone - Apple Support

If you get an alert that says Invalid SIM or No SIM Card installed, follow the steps in this support article:

If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support