Cannot place checkpoints position into Transform array

I am testing a 2D tower defense game in porblem is that I cannot place checkpoint gameobject positions into Waypoints array which is transform array. Drag and drop also not working and is there any alternative way to set these checkpoints in C# script?
I used public Transform waypoints in C# code.

@aungpyae
It appears that you are dragging the checkpoints transforms from the scene into a prefab not present in scene which is not possible. The objects in a scene cannot be linked to a prefab in this way.