My iphone showing sim not support error when I insert my sim in it what should I do now

You are trying to use a SIM from another carrier, other than the one the iPhone is locked to. You would need to contact the carrier the iPhone is locked to and see if you qualify to get it unlocked. See the instructions in this support document, How to unlock your iPhone for use with a different carrier - Apple Support

See if this link helps...

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