iOS will not install on my iPhone 7 Plus

iPhone 7 Plus no longer connects to iTunes on my iMac, and I can’t update to iOS 11 via WiFi. (Error page—will download but won’t install.) Help?

I have tried restarting iTunes on my computer, restarting my phone (at least 3 times), and removing the ios11 update and starting over. It will go through the download process, say “downloaded” on the system update screen, then after pressing “Install Now”, I get an error screen saying “Unable to install Update—An error occurred installing iOS 11.“

Space is not an issue. I’ve closed down all applications. Since iTunes won’t recognize that the phone is even connected to the computer (although I get the charging sign on my phone, so I know the cord works), I can’t update that way.

The kicker is that I got the new Apple Watch 3, and I can’t use it unless my phone is updated to iOS 11!! Gah!

Any suggestions?