An application error has occurred while processing your request

Thanks for posting in the Community, Sergio. Let me provide you with the appropriate steps you may take to narrow down the cause of the error received when trying to send an invoice. 

As you have received a specific error, I'd recommend performing a browser troubleshooting on your end. This way, we'll identify why you've received this error while sending an invoice. 

You may use either of these shortcut keys:

• Mozilla Firefox: press Ctrl + Shift + P
• Internet Explorer: press Ctrl + Shift + P
• Safari: press Command + Shift +N

If it works fine from there, switch back to your regular browser and clear its cache. On the other hand, if the issue persists, try using other supported browsers to narrow down the cause. 

Additionally, you'll want to check this article and learn how to personalize and add specific info to your sales forms in QuickBooks Online: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I'm always here to assist if you need additional assistance managing your invoices and any QuickBooks-related queries. Keep safe.