phone will download new ios but not install it.

I've tried updating my iPhone 6s Plus to IOS 11. It will download the update but I get an error message even I try to install it. I've tried deleting the update and reinstalling it thinking maybe it was because I have poor wifi; but I keep getting the same message. I have enough space for the download and installing too. I'm not sure what the problem is. Has anyone else had this problem? If so, how did you fix it? Thank you!