May I ask what specific issue or error did you encounter when adding VAT to Purchase Order? I'd appreciate any further details so I can provide an accurate solution.
There are times that the browser keeps outdated Internet files and needs to be cleared. This is done once in a while because it can cause websites to receive unusual behavior or an issue with adding VAT to the Purchase Order.
I suggest logging in your QuickBooks account using a private browser. This way, you're able to verify if the issue is caused by a full cache. This might be also the reason that you encountered an issue in adding VAT.
Use these keyboard shortcuts based on the browser you're using.
For Firefox, press Ctrl + Shift + P.
For Chrome, press Ctrl + Shift + N.
For Safari, hold down Command + Shift + N.
If it works in incognito, I recommend clearing the cache of your regular browser. This removes the history or log of sites so you can start with a clean slate. You can check this article to see the steps: Clear cache and cookies to fix issues when using QuickBooks Online.