This is a case where the client's name / email address isn't quite matching between DesignFiles and QuickBooks. If they do not match up exactly, the similarities will be detected so a new client will not be created, but it also will not be able to sync the existing ones.
Double check that you're using the EXACT same email address (and full name spelling) in QuickBooks as you are using in DF for the client on the invoice.
Check your "contacts" in DesignFiles to confirm what you have written and in what format, then check your QuickBooks contacts/customer list for the same client and confirm everything is the same.
Log into your QBO account.
Click on the Sales menu from within the left panel, then select Customers.
On the Customers tab, choose the customer that you want to edit.
Go to the Customer Details tab, then select Edit in the upper left.
Once both contact forms match up completely, the invoice should sync. If they match and you are still experiencing the same sync error, please contact us via the Chat Bubble in the bottom right for further assistance. Please provide a printscreen of the contact card in QBO for reference.