I don’t think you are pulling the right value,
pm.collectionVariables refers to the variables inside the collection, but what is required is the Collection ID.
Take Postman API Collection for Example, down below is the Collection Variables of Postman API Collection.
To get the Collection ID, we check the ‘info’ tab on the right. (i within a circle)
I’m not aware how to retrieve The Collection ID by script, but for the moment you can try copying the ID from here.
Good luck 