iPhone 7 won't update to iOS 15.7

I need to update my iPhone 7 that is running on iOS 11.4 (15F79) to the latest iOS 15.7 (it's supported) to be able to run a lot of apps that I now need to use. However, every time I try to update it, I always get a pop up message that says something along the lines of "There was an error with trying to update to iOS 15.7." I've tried every method and even loaded iTunes onto my computer to try and force my phone to update, but whenever I do that, I get a message on ITunes saying that I need to get the latest version of iTunes in order to update my phone. I just downloaded iTunes onto my laptop a few days ago and when I check what version my iTunes is, it says "This version of iTunes (12.10.11) is the current version." I keep trying to update my phone, and sometimes it does "update" but after it finishes updating, I check and it's still 11.4 (15F79). I've tried everything and even contacted Apple Support. I don't know what else to do. Could the problem be that my iPhone 7 is too behind on the iOS updates? Because I've heard many people saying their phones' battery drain faster when updating, so I didn't bother updating it. But now I'm kinda left with no choice because my phone's battery is already draining fast as I've had this phone for 5 years now. And I need to use certain updates but my current iOS version doesn't support it.