Hi David,
Maybe change the 1099 Vendor box to No, save that change, and then set it back to Yes and save. Can you now enter a Default 1099 Box Number?
I assume you have update rights to that screen.
Perhaps the screen is customized. Is there an * in the title bar of the screen? If yes, try to launch the screen in Standard Mode.
What version of Dynamics SL are you using?