For this one, it usually occurs when something associated with the invoice has been deleted or made inactive on QuickBooks online. This could be the customer, item, or the invoice itself (if it was previously created and synced). Therefore, Sync is unable to update it.
Check QBO for inactive accounts that are associated with your DesignFiles invoice and reactivate them.
Here are the steps to do it for Customers:
Select Sales or Invoicing from the left menu.
Select Customers at the top.
At the top of the customer list click the small gear and select Include Inactive.
Click the customer you wish to restore
Click the Make active button next to the customer name.
Here are the steps to do it for Products and Services:
Go to Settings ⚙, then select Products and Services.
Select the Filter icon.
From the Status ▼ dropdown menu, select All.
Find the product or service that you want to reactivate.
In the Action column, select Make active.
Here are the steps to do it for Categories:
Select Settings ⚙ and then Chart of Accounts.
Select the Gear icon ⚙ at the top of the list.
Select the Include inactive checkbox.
Please check all options against the invoice in DesignFiles to find any item that may have been made inactive that is being used on the Invoice in DF.