I want to help you get rid of the error so you can continue creating invoices, sshaver.
The error 500 usually occurs if there's a compatibility issue with the device you're using and the installed application. This happens to any applications including the QuickBooks mobile and desktop applications.
The QuickBooks Self-Employed mobile application will perform in its optimum level if you have any of the following Operating System:
- iPhone/iPad - OS 11.1 or newer
- Android smartphones and tablets - Nougat 7.1.1 or newer
You can use this link for reference: System requirements for QuickBooks Online, Accountant, and QuickBooks Self-Employed.
I'd recommend updating your device to the suggested operating system. Also, you can uninstall and reinstall the QBSE app. This is to ensure that the app is up to date.
After that, you should be able to create the invoice without a problem.
Keep us posted on the results so we can further assist you if necessary.