Could you please provide the specific error message you encountered? Once VAT is enabled in QuickBooks Online, the option to deactivate it is unavailable. However, you can effectively disable the feature by adjusting your default tax settings. Here's how:
Click the Gear icon and select Account and settings.
Select the Advanced tab from the left menu.
Go to the Accounting section and click Edit.
Change the Default tax rate drop-down menu to Out of Scope of Tax.
Click Save, then select Done.
For more detailed information, refer to this article: Turn off VAT.