Hi meaghan2,
Thanks for posting in the Community!
Have you tried to check with your client to see if they received the emailed invoice?
For 'invoice delivery issue' you can check the email address set on the customer's profile and re-enter it. On the other hand, click on delivery issue message to open more details and click Resend invoice.
You can test this out by temporarily changing one of your client's email to yours and send an invoice to see if you get the same error.
For more information, you can also check on this article: 3 solutions for you when customers aren't receiving your emails.
Feel free to drop a comment below if you need further assistance and I'd be glad to help.
