Hi there, daniel8.
I know this isn't the experience we want you to have. As I've checked on the current investigation, I do see here that our engineers are still working on fixing the issue. Rest assured they aren't stopping until you'll be able to send invoices to your customers.
If you haven't been added to the list of affected users, I encourage you to reach out to our QuickBooks Online Support. They'll be able to pull up your account and add you so you'll receive an email notification once it's fixed. Please follow the steps provided by LieraMarie_A on how to reach them.
In the meantime, you can consider manually downloading your invoice and sending them outside QuickBooks. For your reference, you can visit this article for more details: Download and save invoices, other sales forms, and statements as PDF.
I've added these links if you need to customize your sales forms and for the steps on how to record invoice payments:
You're always welcome if you need further assistance with sending invoices. We're always here to lend a helping hand.
