Well, what you are trying to do is not possible, because you can't refer objects by their name.
By selecting multiple objects I didn't mean "at once", I meant selecting instances of different objects.
Say, you can select either Block sprite, or Tree sprite or Rock sprite.
And understandably you don't want to create 3 almost identical events. (if selectedObject="Block" then set position to block.. if selectedObject="Tree" then set position to Tree.. if selectedObject="Rock" .....)
So the workaround would be adding all three sprites to a family SelectableSprites. And then do what I suggested in my previous comment - pick family member by UID and set position to family instance.
And one more thing - BlockMoveable(21) means instance with IID=21, not with UID=21
https://www.scirra.com/manual/130/common-features