The "Tax Rate Can't be Determined" error message means that the Tax Rate on the order can't be matched up with any of the Tax Rates that exist in QuickBooks Online. You can find out which rate didn't match up by appending /admin to the app URL and selecting Details.
Failure data including the Internal Message will be displayed.
In this case, the rates "Broward County Sales Tax" and "Palm Beach County Sales Tax" have the same agency and rate, and there was nothing in the name that indicated which rate to use.
To solve this, you can use different tax agencies or name the tax according to the location. If a merchant has complicated and non-conventional tax setup the app may fail to find a tax rate.