Why can’t I update to the real iOS 15 I been had the beta one but now my phone won’t update

You need to unenroll from Beta in order to update to the public release.

See this --> https://beta.apple.com/sp/betaprogram/unenroll

If you don't have a computer you have no business testing beta versions on your device. You will need to use a friend's computer that has a recent Mac or Windows computer with the laster version of iTunes or make a Genius Bar appointment and have them restore it as a new device.

Too bad somebody didn't get up in your business before you started doing this. You could just leave it with the beta version and sell it. You can also try (if you haven't already) doing an Erase All Content and Settings and then an OTA Restore as a new device and see if that gets you to an RC build number.

Have a broken windows computer and a Chromebook isn't doing you much good.

Go to Settings > General > About > Version and post what version you are running and the build number.

That is a beta version. After you removed your beta profile you did restore your iPhone as directed?