Any expression applied to a position property must return an array. From the little I can see of the expression in your screenshot it just points to a layer. I'm assuming there is a layer in the timeline named "Targget" and that you want to use some property in that layer. At the very least your expression should read: thisComp.layer("Targget").poisiton
You should study the After Effects expression reference and guide. You'll find it easily by typing expressions in the search help field at the top right corner of AE and following the links.
I'm also not sure but you may have to write some of the expression in the same language your are using in your OS.
Thank you for your answer !
Later I found that I wasn't using the expression on the position of the Target, as you said, but just on the Target himself.