Try making sure your feature manager tree is not rolled back. You cannot modify the material of a multibody part while the feature manager tree is in rollback state. When rolled back past features, the cut list items that result from those features are also unavailable. Therefore, it is not possible to automatically sort the cut list in this state.
Since the application of a material to a cut list body requires automatic sorting of the cut list, this functionality is disabled until the full cut list is available again