I've got some steps to help you send your invoices in QuickBooks Self-employed (QBSE).
The error usually happens when the email address format is incorrect. To fix this, we can re-enter both your email address and your customer's email address on the Invoice page.
Here’s how to re-enter your email address:
Go to Invoices from the left menu.
Click Create invoice.
Hit Edit work info.
In the Email field, re-enter the email address.
Click Save.
Now, let’s re-enter your customer's email address on the same page:
Click the Edit customer info.
In the Email field, re-enter the email address.
Click Save.
Moreover, if the troubleshooting steps above do not work, you can resend the invoice using a new incognito or other supported browser.