Non-Taxable Invoice requires Exempt Sales Tax Code
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:
It's also possible your QuickBooks Online file in Mobius has lost it's Locale. Please review the following knowledge base article to correct this issue.