Hi Mel1958,

Welcome to Community!  Having the ability to send invoices to your customers in QuickBooks Self Employed, is a vital part of getting paid on time.  I'd be glad to help!

If you haven't done so already, I recommend logging into your account from a web browser.  Once you're logged in, review the customer profile to ensure the email address is accurate with no spaces.  In addition, I suggest reviewing your billing and subscription page, to ensure your account is active.  If you experience the same result, please reach out to QuickBooks Self-Employed Support, for additional assistance from their end.

Feel free to reach back out with any other questions.  We'd be happy to assist!