Hello everyone. 

We're aware that some QuickBooks Online (QBO) users are currently having a problem sending invoices to their customers. Our engineers are working to fix it as soon as possible.

As a workaround, you can download the invoices as a PDF and attach them to an email outside QBO. Please know that if you opt for this workaround, you'll need to manually apply the payment to the invoice once you receive it from your customer. 

We’ll share updates as soon as we learn more. You may also check for updates on your end from time to time through this link: https://status.quickbooks.intuit.com/

Thank you for your understanding while we're working on a fix.