Javascript and PHP browser issue

Description

I am having issues with my opencart website. Some customers are experiencing javascript errors that won't let access the checkout page in Chrome or Edge. Also some the buttons and links throughout checkout process don't work either for these customers as well. I think it's it's a javascript error for certain browsers.
So I need an expert php and javascript developer to test, fix and retest on all browsers on both mobile and desktop.
All 'Continue' buttons and checkout links must work in all browsers across all devices.
Developer must be 5 star Top Cert only and expert in javascript and php and understand server and website caching.