Can't connect prefab to prefab

Hello, I'm following a tutorial and i'm facing an issue.
In the tutorial, he drags Path(0) that contains children(waypoint(0), ...) into the project window to create a prefab.
In the project window, he can access to the children by clicking on an arrow that is displayed on the prefab and can select the children to drag them into the List of a prefab.

But in my Unity, I don't have that arrow. I can't select the children, thus when I drag Path(0)it fills the List with Path(0) and not his children.
He uses Unity 2018 and me 2019, but how can I achieve the same thing ?
His Unity looks like this :

View attachment 422974
And mine :
View attachment 422977
Mine
Thanks for your help

 

What you can do instead may vary based on what was done in the tutorial, but you can likely double-click on the Prefab in the project window to open it for editing and then drag the children from in there. If this is still giving issues, please link to the tutorial in question.