Hello and welcome to the Community, kellyspider1973-
We're aware that some customers are receiving the above message when attempting to save invoices in QuickBooks Online.
At the moment, the error appears to be intermittent and may self-resolve by creating a new invoice. Rest assured; our team are looking into the cause to ensure it does not continue.
If you would like to be added to our internal investigation for updates, please contact our support team directly quoting the reference INV-103515. Thank you!