Describe the actions you are taking in trying to drag an object from the library onto the the stage. It should be as simple as click down on the symbol's icon in the library and move the mouse so the cursor/object-outline is over the stage and then release the mouse.
I think this can happen while dragging the items from library panel to the stage when the selected layer is locked. i.e. the layer which is active might be Locked in your case. Make sure the Layer selected / highlighted in layers panel is not Locked(there should not be a lock icon next to it in layers panel for the selected / highlighted layer, if exist just click on the lock icon to remove it)
ps: please mark this post as Answered if this is of help to you