Welcome to the Community. I'd be happy to help you go to the Customers tab in QuickBooks Online.
Currently, we don't have any similar reported cases about being unable to go to the Customer section. It's possible that a web page issue is preventing you from doing this.
Let's try performing some troubleshooting steps. To begin, let's start switching to incognito or private browser to determine if its a browser related problem.
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift + N
After that, try going to the Customer section once more. If this works, switch back to the regular browser and clear its cache and temporary Internet files. It could be the browser is holding too much data, and this is what's causing unexpected behavior while using the product.
Meanwhile, here are some related articles about the Customer section in QBO: