I can share some clarification about the error message in QuickBooks Online.
Please make sure to add tax codes to each of the items you enter when creating transactions. If the one you're leaving blank is non-VAT, it should have codes like No VAT (0%) or Exempt (0%).
If you're unsure of what to select, I recommend consulting an accountant or a tax professional for expert advice. Also, you can choose Exclusive of Tax, Inclusive of Tax, or No VAT depending on the items you enter (see screenshot below).
I've included some articles you can use to handle VAT, like how to apply them in sales forms: