If I were you I’d go right into the storage menu in the Settings app and offload every single large app that you can (you can immediately reinstall them after the update, and they’ll remember your data), and if Messages is listed as taking up more than a gig of space, I’d delete any large attachments you see there, like videos you’ve sent to friends or family in the past. This will likely free up a lot more storage space to make sure you have more than enough. I’m talking if the iPhone says you need 11 gigs, try to give yourself 20+ to be safe. If you do this and it still doesn’t work, I’d recommend you contact Apple because it sounds like that could be a problem.