Hello alekskn99,
I went ahead and made a quick example that should get you pointed in the correct direction. I hope that this information helps.
Example:
In this example I have used a “Get all actors of class” node on begin play within the level blueprint. This means that when the level starts this blueprint will find all of the “Keys” in the level. From there I got the length of the array that is produced and subtracted one (array index starts with 0) and used that as the max for my “Random integer in Range” node. Finally I get my random actor “Key” and set a boolean that I added to the actor.
Make it a great day