Error

'NoneType' object is not iterable

Cause

The invoice in question may have a tax exempt line in it. It is possible that the tax exempt code mapping under the Taxes tab of the QuickBooks Online Configuration is mapped to a non-existent tax code in QuickBooks Online. 

Resolution

Replace the erroneous mapping with an existing tax code. Please review the following articles to resolve the issue.

1. Tax mappings
2. "Non-Taxable Invoice requires Exempt Sales Tax Code"