[John Doty] “The menu is 16:9 but my footage is 4:3, is that the problem?”
I believe that what should happen is the 4:3 footage gets stretched to 16:9 to fit the menu frame. But it may be this mismatch that is triggering your error.
In Encore, motion menus have a video background that fills the entire menu frame.
Create a new menu or asset so that the aspect ratio of your menu and video background match exactly: both 16:9 or both 4:3.