‘Sim not supported ‘ error tried reset and all nothing works . please help me fix.

The phone is carrier locked. If it was working previously, that indicates the device was previously hacked to illegally unlock it.

Only the carrier it is locked to can remove the lock. Apple does not have the legal right to remove the carrier lock.

If you don't know what carrier it's locked to, call AppleCare and give them the serial number. They may be able to tell you.

You don’t need to contact the cellular carrier for your area, you need to contact the cellular carrier the phone is locked to. You need to ask a real person at Apple Support who that carrier is, then you must contact that carrier. Sometimes the phone will tell you if it is running iOS 14. Go to Settings/General/About and it MAY display the name of the carrier. While there note the model number (not serial number) and post it. That can tell at least what country the carrier is in.

Use the Get Support link at the top of this page to contact Apple and ask.

Your phone is locked to a carrier. If it worked in the past perhaps it was hacked to bypass SIM lock, but that hack is not permanent. You need to find out the carrier it is locked to and ask about their unlocking policy.

The phone needs to be jailbroken to bypass SIM lock, but most evidence of that can be removed. Go to Settings/General/About and see what it says for Carrier Lock (If you have an older version of iOS you won’t have this field). Beyond, that, use the Get Support link at the top of this page and ask an Apple advisor.

Learn what to do if you see a SIM not supported alert.