https://us-kb.sage.com/portal/app/portlets/results/viewsolution.jsp?solutionid=240513185155153

This order only has standard inventory items.

Tax amount is $0.

No customer tax exemptions.

Kevin gave you the link to a defect in Sage 100 doesn't matter what taxes are, what inventory items, customer tax exemptions.  It is a bug that was fixed per Kevin's link above.  It was if you had a miscellaneous item, comment on the first line of the sales order.  We fixed for versions 2024.1 & 2023.3.

I appreciate the link, but this is an Avatax issue and has nothing to do with the linked article.

The “Invalid Tax Calc” flag might be set to “Y”. Recalculate Tax or DFDM the field in SO_Header.