forgot iphone passcode and only have chrome book

You can’t use a Chromebook. The only way to unlock it is to use a computer running iTunes or a Mac running Catalina through the Finder.

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

You can borrow a friend or family member’s computer or make an appointment at an Apple Store and ask for their help.

iTunes will not run on a Chromebook. It won’t work.

you'd have to get iTunes running on the Chromebook and I'm not sure if it is compatible..