Hello there, Noel. Let's make sure to fix the error while adding a new customer to your invoice in QuickBooks Self-Employed.
Before that, may I know what specific error did you encounter? This will allow me to identify what resolution that can fix this issue.
An overloaded browser may sometimes cause problems with loading or formatting issues on a website, like missing images or buttons that don't respond when clicked. It also includes errors that you're getting when trying to do some processes in QuickBooks.
You'll want to log in to your account using a private browser (incognito) to check if you're still seeing the error. I've provided the shortcut keys below so you can follow them:
Safari: Control + Option + P
Google Chrome: press Ctrl + Shift + N
Microsoft Edge: press Ctrl + Shift + N
Firefox: press: Shift + CTRL + P
If you're able to use your (QBSE) without issues, return to your default browser and clear the cache so you can work with a clean slate. Your cache and extensions are probably interferes the scripts QuickBooks is trying to run on the page.