Thanks for coming back to the Community, sarahpm.
I appreciate you for sharing your experience about the delivery error of emailed invoices.
You're right that the closest report we can get is the Audit Log. From there, you'll have to open each invoice to see the delivery error.
We're always working to make QuickBooks Online exactly what you need for your business. You help us decide which features to build, and what improvements should be made to our platform.
As always, you can check regularly for product updates and feature enhancement through this link: QuickBooks Blog.
That information should get you on the right track. I'm here to help some more if you have other questions about QuickBooks Online. Have a good one.