Run an Autolaunched Flow from a Custom Button - {!campID} does not exist

To compose my Campaign Button URL, linked to my AUT flow Create_Child_Campaign, I had to insert variables from my flow. 

Here is the order : 

• Text: Enter a relative URL using the Create Child Campaign flow’s URL and these parameters in this order:
  • Set campID to the Campaign ID merge field
  • Set campName to the Campaign Name merge field
  • Set campOwner to the Owner ID merge field
  • Set campStartDate to the Start Date merge field (set the variable to be equal to
  • {!TEXT(Campaign.StartDate)} to format the date correctly)
  • Set retURL to the Campaign ID merge field

Below, my URL with the error I don't understand (the variable campID does exist in my flow).

I tried it out on two different playgrounds, and I received an error on the campID filed twice. I can't pass this badge because it. 

Thanks for any help ! 

#Flow