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We can perform some basic troubleshooting steps to fix this, jwykle.
It could be that the accumulated cache in the system is the reason why you're not able to save the invoices. You can follow the steps below if you're using the QBO Desktop App for Windows:
If you're referring to the QBO mobile app, please follow these steps.
In the meantime, you can access QuickBooks Online through a web browser then create the invoice from there.
In case you need a report that lists all invoices that are due on a date range, you can visit this article in customizing the Transaction Detail By Account report.
Let me know how it goes in the comment section, as I want to ensure that you can successfully save your invoices.