Hi there, Comfortjames.

I can see how important it is to send your invoices to your customers on time from QuickBooks Desktop. 

There are possible reasons why you are unable to send an invoice in QuickBooks. One of the reasons is that the QuickBooks Desktop version you are using is outdated. Make sure to update QuickBooks to the latest release. 

The other reason is that something in your web email security settings is preventing you from sending forms through QuickBooks. To fix this you'll need to review your email provider's security settings and other filter preferences.

If you're using Gmail, here's what you can do:

1. Enable access for less secure apps.
2. You can also set access preferences for specific apps such as QuickBooks.
3. Return to QuickBooks and email yourself a test transaction. If it still doesn't work, you can disable the 2-Step Verification feature in your Gmail account.

Also, you'll be prompted to authorize QuickBooks Desktop whenever you send a transaction if you've set up Gmail to use enhanced security. You can use this link as a reference: Reauthorize QuickBooks Desktop to keep using Gmail

If you're using Yahoo! mail, here's what you need to do:

1. Go to your Yahoo Account Security page.
2. Turn on the Two-step verification and Allow apps that use less secure sign in features.
3. Go back to QuickBooks and email yourself a test transaction. If you still can't send emails, use Yahoo's generate third-party app passwords tool. It creates a random password you can use to connect QuickBooks. You only need to do this once.

For complete details on how to resolve web email issues, you can check out this article: Fix webmail password issues in QuickBooks Desktop.

Once you've successfully sent your invoices, you can visit this resource to learn more about processing customer payments: Take and process payments in QuickBooks Desktop

Feel free to return to this thread and add some details or ask any follow-up questions about invoices. I'll be here to assist. Have a wonderful day!