I'm happy to see you posting here in QuickBooks Community. I'll provide steps to ensure that the invoice will send to your customer. Can you please provide the specific error you're getting? I'd appreciate any details so I can provide an accurate solution.
For now, you'll need to make sure that the field doesn't have any extra characters or spaces before, within, or after the email address of your customer. This is to make sure you enter the correct format of the customer's email address. If the issue persists, I'd suggest going to the Account and Settings page. From there, you'll need to check and make sure that your company email doesn't have any extra characters or spaces before, within, or after the address.
Click the Gear ⚙️ icon.
Under Your Company, choose Account and Settings.
Go to the Company tab and choose Contact info section, select Email.
Clear the Company email field and enter the desired address, even if it appears correct as is. (Note: Be sure not to use any extra characters or spaces before, within, or after the address you enter).
Hit Save and Done.
Once done, send yourself a test transaction. If you still don't receive the email, try using a different email address that has a different domain (for instance, if you were using a Gmail email address, try a Hotmail or Yahoo address instead).
If the problem continues to occur, I'd suggest accessing your account via a private or incognito window to check if this is a browser-related issue. At times, data stored for a long period causes unusual behavior to the browser.
You can use either of these keyboard shortcuts:
For Google Chrome browser: Ctrl + Shift + N
For Mozilla Firefox browser: Ctrl + Shift + P
For Safari browser: Command + Option + P
If it works, then your main browser may have too much historical data from previously visited websites that cause this kind of behavior. Just go back to your regular browser and clear the cache to delete those temporarily stored files and browsing history. You can also use other supported, up-to-date browsers as an alternative.