It’s possible that this is web page cache issue since you're unable to see the Billing & Subscription tab in QuickBooks Online (QBO). To verify this, we can try using your browser's incognito. This feature doesn't use the existing cache files, which makes it a good place to know if there's an issue with the browser.
You can use the following keyboard shortcuts for your reference:
If you're able to upgrade successfully using the incognito feature, I suggest you go back to your main browser, and then clear the cache. This step resolves most browser-related issues. Alternatively, you can use other supported browser.
You know where to find me if you need help with anything QuickBooks. I'm always happy to assist. Stay safe and have a great day ahead!