If you do not have or have not created any Sales Tax Rates in QuickBooks Online, that must be done before a rate is selected within the QuickBooks Sync Settings in Unify
How to create a sales tax rate in QuickBooks Online
1) In the QuickBooks Online (QBO) menu select Taxes and Sales tax and you will be taken to the Sales Tax Center in QuickBooks Online. Click on the Set up Sales Tax Rate button.
2) Select or de-select the various options to setup your specific tax rate that is used within your online store. Click Save.
If you have multiple tax locations or multi-location taxes in your online store. They can also be created via this process as well.
3) After the tax rate has been created, you can adjust it or add more within the Sales Tax Center in QuickBooks Online.
Once all of your tax items are created in Quickbooks, go back into Unify and click the refresh button within the Settings >> Sync Sales > Sales Tax.
Once the data refresh is complete, you will be able to select your new tax items to configure the taxes area of the QuickBooks Online sync settings.
In QuickBooks Online
After you have posted the order to QBO using the QB Sync Button, the order will retrieve the sales tax rate and display discrepancies (if any) in the sales tax taken in the online store vs. the sales tax rate used in QBO for online orders.