Currently, there's an ongoing investigation (INV-82020) about undeliverable invoices in QuickBooks Online (QBO).
You can resend the invoices after 48 hours. If the same error persists, you can download the invoice and send it to your customer using your email. Here's how:
Go to Sales and select Invoices.
Look for the invoice and click the Edit icon beside the Action column.
In the invoice form, click the Print or Preview tab. Then, pick the Print or Preview option.
Once done, click the Download icon below the shown page. Then, send it to your customers using your email.
Also, I recommend contacting the QBO Care Team to add you as an affected user. You can further help fix this issue by sharing your experience while sending your invoice.
If you have additional questions about sending your invoices, click the Reply button below. We'll assist you as soon as possible. Keep safe, and have a good one.
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