I can’t update my iPhone 6 in iOS 15. Please help!!

An iPhone 6 can't be updated beyond 12.5.5, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

I can’t update my iPhone upto iOS 15. Why?

The simple non-technical answer........your iPhone is too old. You need a new iPhone.