SIM card not supported

Only the U.S. carrier that locked the iPhone can do that and since you are not in the U.S. your chances are zero.

Ask your cousin in the U.S. to apply for the unlock and if s/he's qualify.

Read here => How to unlock your iPhone for use with a different carrier - Apple Support

Hi,

Your device might be under carrier lock. This should be unlocked via;

1. Try new SIM or
2. Restore device via iTunes or
3. Remove SIM lock via seller

Either speak with Apple Support for further support

i bought an iphone 8 that is locked and will not support my carrier. how do i fix this . i need my phone.

If it is carrier locked, you will need to contact the carrier to get it unlocked and to see if you qualify to get it unlocked.