To set this up go to Set Up QuickBooks Online > Select your company > Taxes Tab > set "Exempt Sales Tax Code"
Please note that different QuickBooks Online country versions require different Tax configurations. If your QuickBooks Online account does not require Tax Exempt, then this message means that the QuickBooks Online Config has lost the Locale, i.e. the country version:
To resolve this, please re-save the configuration, i.e. go to Set Up QuickBooks and the corresponding Config, and click Save. This will re-populate the Locale: