I can't update my iPhone to iOS 15.8

I put my working SIM in, and I can browse the web on cellular but am also having this exact problem and error message. My iPhone 7 is on 15.0.2, and won't install 15.1, which is the only update it sees.

Try updating using your computer -

Click on this link for instructions on how to update your phone using a computer

Update iOS on iPhone - Apple Support

You need a cellular network connection to activate the iPhone after an update.

I fixed this by connecting to my Mac and upgrading iOS in the Finder.