I can help you send the email invoices successfully in QuickBooks Online (QBO), Porter.
To start, may I know if you've encountered any specific error messages while trying to click the Send button? This way, we can check for any reported issues related to your concern to give the most accurate resolution.
In the meantime, it's important to double-check that the email address is accurate. You can also attempt to send an email to yourself to confirm that it's not possible to send when clicking the Send button. However, if you receive the email, we can check whether if this is a browser-related issue. I recommend logging into your account using an incognito window to clear the corrupted browser cache using the following keyboard shortcut keys:
Also, you can check in the Audit log feature to trace all activities in your QuickBooks, such as user sign-ins, changes to QuickBooks settings, edits to customers, vendors, employees, and payroll submissions, and delete/edit any transactions. This allows you to check the status of the email (whether it was delivered or not) and ensure all activities within the program are accurate.