my iphone 6 plus is not locked to any carrier, however it shows sim not supported i dont know what is the problem please help

The problem is exactly what it stated on your screen. The SIM card you have inserted is not supported. This means that your iPhone is LOCKED to the carrier that it last worked on. I understand that you believe that your iPhone is not carrier locked, however, the screen error message is indicating otherwise. I recommend that you contact the carrier that the iPhone last worked on and ask them what are the requirements to get your iPhone carrier unlocked so that it can be used with other carriers.

Axel F.