I've just tested the Order With no Target for unloading bunker and it woks perfectly. My suggestion is to check whether the unit that you issue order to is indeed a bunker.
It's not clear what "Building" is from your screenshot, but "Attacking Unit" does not look like a bunker at all, as the bunker itself does not attack (unless you've changed it).
You can kind of debug it using "Text Message" with "Name of Unit Type" just to be sure.
I've just tested the Order With no Target for unloading bunker and it woks perfectly. My suggestion is to check whether the unit that you issue order to is indeed a bunker.
It's not clear what "Building" is from your screenshot, but "Attacking Unit" does not look like a bunker at all, as the bunker itself does not attack (unless you've changed it).
You can kind of debug it using "Text Message" with "Name of Unit Type" just to be sure.