And how can I use the instantiate action to have the prefab instantiated at a random place ?
I'm opening a separate topic for your question. Please, make sure from now on to post each question in a different topic. This helps keep things organized.
Regarding your question, you can use a List Variables pre-filled with random positions (use either Vector3 or Game Object types). Then, using the "Instantiate" action, set the position's source as a List Variable. You'll be able to choose "random" from the list of options available.
Your "random" option is finally not doing the job : I have several objects instantiated at the same random position.
Well, the "random" pick from a List Variable is just random. If you want more fine grain control you can always create a custom Action similar to "Instantiate" Action and use the Random.unitInsideCircle property.
Despite this, with the current system it should work just fine. However, bear in mind that if you have 3 possible positions and you instantiate 2 items randomly, there's a slight chance that these will appear at the same position. To solve this, either increase the amount of slots in the List Variable or create a custom Action (if you can code) that completely randomizes the position.
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