Welcome to the Community, @lindsey11. I'm here to assist with the 404 error that you're getting.
Just to clarify, are you selecting the "+/=" key next to Backspace? If so, it would be helpful to know what happens when you try selecting the "+" key that's on the numeric keypad.
In the meantime, I'm including an article with information on keyboard shortcuts in the software: Keyboard shortcuts for QuickBooks Online.
I'll be keeping an eye out for your response and look forward to getting to the bottom of this. Take care.