Invoice wont sync to Quickbooks because of the completed on date?

If you have an error telling you "Completed on can't be blank", this typically means that the completed on date is in fact blank OR it is later than what you have listed for the Invoice date. This confuses Quickbooks.
You can resolve this by first making sure the job is marked complete. Use the View button and choose Job to confirm this. From there, you can also "Edit Job" to change the completed on date. Use a day that is equal to or prior to the displayed invoice date.