Unable to update my iPhone 6 Plus from 12.5.5 to iOS 15.1

Somewhat speculative, but I am glad you agreed with what I posted hours earlier about 12.5.7 being the only version currently available for that model iPhone. However, we don't know for sure Apple won't suddenly drop 12.5.8. 12.5.7 was an unusual release itself, coming out several months after iOS 16 and out of the general pattern of Apple only releasing updates for the last two iOS versions. I was very careful not to assume Apple won't surprise us again.

The best that the iPhone 6 Plus can do is IOS 12.5.7.

That is the end of the line.

If you need to run higher iOS versions, you need a new phone.

Only iOS 12.5.7 is currently available for an iPhone 6 Plus