Error "SIM not supported" on my iPhone

The phone is locked to a carrier other than the one you are attempting to use. Apple cannot remove a SIM lock. You will need to contact the carrier that the phone is locked to. If you just purchased this device from a third party, return it as there may be nothing you will be able to do.

How to unlock your iPhone for use with a different carrier - Apple Support

If you did purchase this device from a third party, review the information provided here:

The All Too Common SAD Reality of Buying … - Apple Community

From the message provided on your iPhone, it seems that the SIM in your iPhone is not supported due to SIM Lock. That means you can only use a SIM with the network it is locked to. Go to Settings > General > About > Scroll down to Carrier. There it will tell you what carrier your iPhone is locked to. If you happen to purchase this iPhone used, you were scammed and you probably won't be able to get it unlocked. Carriers usually only unlock it if you have payed off the standing balance or finished the contract, and even so that requires the personal information of the original owner. Try getting a refund if that's the case.