When we go to create an invoice, under Sales Tax Details it says "An application error has occurred while processing your request" and we are unable to create an invoice

Hi, good thing you posted your concern here.  

I want to make sure you can create an invoice in your QuickBooks Online (QBO) without getting an error.

You might be experiencing web browser issues. The system occasionally behaves strangely when the browser frequently accesses pages. It could be due to corrupted cache files in your browser. 

Let's perform a few troubleshooting solutions to isolate the issue. I suggest using a different browser or a private window to help you fix this so QuickBooks Online runs more smoothly and effectively. You can use these keyboard shortcuts:

• Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Safari: Command + Shift + N
  
  

Once done, please create an invoice again. If it works without error, go back to your regular browser and clear the cache to fix the unexpected behavior in the program. If not, use other supported web engines. The current one you're using may have a temporary issue with QuickBooks. Utilizing a new browser allows you to get back to working order.

I've also attached a link you can refer to learn how to personalize invoices and other sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

If you have additional questions about creating invoices, click the reply button below. I'll be here to lend some help. Take care.

I have already tried this and I’ve tried 3 different computers. It is not a problem with my computer, and it has worked perfectly fine until today 

I have already tried this multiple times and I’ve tried 3 different computers. It is not a problem with my browser or computer - and a quick google search says others have had the same issue with no resolution

I have already tried this multiple times and I’ve tried 3 different computers. It’s not an issue on my end 

Hello there, tbren100.

I appreciate you for trying the recommendation shared by my colleague and sharing the result with us. 

Using a private window and other supported browsers to access QuickBooks fixes odd behavior and error messages. Since the issue persists, I'd recommend contacting our Support Team. They'll help us figure out the reason why it is still happening.

1. Within your QBO account, go to Help in the upper-right corner.
2. In the Search tab, click Contact us.
3. Enter a brief description of your concern.
4. Click Continue.

I've gathered some articles where we can get details about invoices and payments:

• Add, organize, and send invoices.
• Topics about your company's income and customers. 

You can always post in the Community if you have other questions, especially about creating sales transactions and sales. Take care and stay safe.