When I go to Appearance –> Customize, I get the error page.

I’m not sure what theme you might be using, but possible that’s functionality has been removed or tweaked with your specific theme? Alternatively, could be a plugin causing some conflicts here and causing the customizer to break? You might try disabling all plugins and seeing if the customizer works, and then if it does, re-activate plugins one by one to see if there is one causing an issue. I would start with that to see if that resolves the issue.