Either I don't understand what it is you want to accomplish, or there is some fundamentals that I need to go over.
Binding: This is the association between the control (i.e. a drop-down list box) and and XML node (i.e. field) in a DOM. Most commonly, this binding occurs on a Main DOM field, as these are the ones that store the values in a way that can be saved. This binding might also occur on secondary data source fields if you are using Web service, or database or for other special purposes.
It is never appropriate to bind a drop-down control to a repeating list, which is why you get that error.
My guess is that you don't really want to bind the control to the list, but that you want to "populate" the control using the list. The binding is for the stored value (i.e. where you want to record the selection from the list that the user made).
If you really want to bind to the repeating list, then where are you populating the drop-down from? More likely, the list is for population, and the stored value (binding) is somewhere in the Main DOM.