Unable to update iPhone to iOS 15.1

15.1 is no longer available

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3.1

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK)

If you are trying to update your iPhone 6 Plus as stated in your tagline, it can only be updated to iOS 12.5.7 and no further due to hardware limitations. Consider looking into a newer iPhone that can still run a newer, supporting iOS.