When I tried to import my IIF file in QB it returned error 21 or a warning: "The tax field cannot be left blank, even for non-taxable customers and sales which have no taxable items, so non-taxable sales will be correct for this district."

You may also have received this warning: "Error on Line XX: Can't record invalid transaction."

In most cases this is due to the fact that in QB under Edit > Preferences > Sales Tax you are set to receive sales tax. In most cases swimming teams are set to No for sales tax so our IIF has been set to this same parameter.

If you cannot locate the sales tax preference please check your help on QuickBooks as versions can change.

Set this to "No" if it pertains and you should import correctly.

See Also
How do I create my TU chart of account created in QB?
QuickBooks CoA shows 0 or negative amount and account type set to bank