I'll help you fix the error message you've encountered when adding Value Added Tax (VAT), Anthony.
Before anything else, please know that the system automatically calculates the sales tax rate for sale based on the following scenarios:
Otherwise, you'll want to ensure you've selected the correct VAT for each line item.
Also, you can perform troubleshooting steps to determine if this is a browser-related issue by accessing your QuickBooks Online (QBO) account using a private window. Thus, too much historical data from the browser causes discrepancies that affect the performance of your QuickBooks.
You can use either of these keyboard shortcuts to open an incognito window, depending on your browser:
If it works, you can return to your original browser and clear its cache to start fresh. You can also use another supported and up-to-date browser to narrow down the results.
In addition, I'll attach this article to learn more about how QBO automatically calculates VAT when you make a sale: Learn how QuickBooks Online calculates VAT.
Also, you can visit this resource for in-depth information about setting up VAT: Set up and edit VAT settings, codes, and rates in QuickBooks Online.
Feel free to reply on the thread again if you need more help managing your taxes. Stay safe always.
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