Are you trying to place a policy in an empty or a filled slot?
If filled, you have to empty it first
It is a all or nothing thing.
I can drag&drop with replacement or I can not drop because it is not snapping in.
Interestingly it is decided on game start if that works or not.
So if it is after starting a session you see if that is working:
If yes, you can continue using it.
If no, you need to start a complete new game.