The Pro Tools error message "the current session has an output bus mapped to this output and it may not be remapped to an object" is seen when trying to map a track to an object that already has the track output using the same physical output.  


If a bus is mapped to an output and mapped to an object is will not be available as a track output.   It a bus is not mapped to an object it will be available as a track output physical output but not as a object selection.


The bus/object toggle cannot be used with the same bus being used for both.   The way around this is to select another output that is not mapped to an object for example a bus used for a bed.