These error messages can be avoided by editing your Sales settings in QBO. The error messages would contain:
"Not Able To Update Preferences", or
“...Discount Line Is Missing”
To set things right, follow these steps:
Click the Settings icon in the top-right corner of your QuickBooks screen
Under Account and Settings > Sales > Sales Form Content, find the off/on switches.
Make sure “Custom fields”, “Discount”, and “Deposit” are set to "On".
A more complete view of the Account and Settings page for the Sales section will now look like this:
Now you will be able to update invoices that have already had payments assigned to them or those that have overall order discounts!