Let's perform some troubleshooting steps so you can add taxes successfully in QuickBooks.
Before we start, did you receive any error message when trying to add taxes? Any additional information would be greatly appreciated.
Let's try logging into your QuickBooks account using a private browser. There are times that the browser stores frequently-accessed data, thus causing websites, like QuickBooks Online (QBO), to act weirdly.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and try to add taxes again to double-check. For your reference, you can check out this article: How do I Set up Sales GST/VAT Rates?