If you're creating service items to map on the Line Item or Taxes tab, you'll need to set them to non-taxable before you can resume or start your sync so that QuickBooks Online doesn't tax them.
Depending on your QuickBooks Online version, you may just have to uncheck the "Is taxable" box or select "Nontaxable" when you edit the service item.
You'll see either this
or this for your service item
Make the proper changes and then save.
You can now go back to SellerZen and resume or start your sync.