I appreciate you for reaching out to us here on the Community page. I'm here to help you with saving your invoices in QuickBooks Online.
To get started, let's make sure that all fields and information are added to your invoices before clicking on the Save or Save and Close button. If everything is correct and the error continues, let's proceed with saving your invoices through a private window. In most cases, unexpected behavior with QuickBooks happens due to the stocked cache on the regular browser you're using.
To save you time, use either of these keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Safari 11 or newer: ⌘ + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
If you're able to save your invoices with no errors, I recommend going back to your original browser and clear its cache to start fresh.