My iPhone currently has the error message for the support.apple.com/iPhone/restore, but I can’t do it correctly because my off button is broken

Not quite sure why you want to bring to life a phone with no power button as it will still be unusable.

You have two options, buy a new phone or you can arrange a service or repair

https://support.apple.com/iphone/repair/service

Anything other than Screen or Battery may be a replacement phone but at a less than retail price.

You can see additional service information within the following article: 

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

Sorry, first you need to get your iPhone fixed.

Use Get Support link at the top right of this page for options.