This is the last step of Sync Sales Settings. It is here that you decide how would like the taxes charged in the online store to be recorded into QuickBooks online.
Under this option, Unify will ask you to Select a Sales tax item which you can select from the drop-down. If nothing appears in the Sales tax item field, or if your choice of sales tax item is not there in the list, tax items may need to be created in QuickBooks Online.
Here is how to create a Sales Tax item in QB and bring it over in Unify: