First, the point of sketchPath(), the function, is to build a valid path out of the parameter you give.
So instead of:
findPathScetch = (sketchPath("") +"data/target.app") ;
it is better to do:
findPathScetch = sketchPath("data/target.app");

(BTW, that's sketch, not scetch, but I acknowledge the name is odd (for a French like me...))

Second, why do you expect the path application.macosx/OpenApp.app/Contents/Resources/Java/data/target.app to be in the sketch folder? Looks like a system path, should be independent of the location of the sketch. The final data/target.app looks odd too.

Ok, sorry for the S"c"etch...may be the first day of summer is too hot for my brain !

So my idea is make a launcher app, call the other applications. I must find a way to give the universal path to the launcher. I think put the app in data folder of the launcher is a good idea. Work when the launcher is a sketch but when it's app, nothing happen, so I change the path but nothing change..the laucher make nothing.

PhiLho, application.macosx is a directory created by exporting a mac application.

I try something with selectInput... but now nothing work, no PDE way, no APP way.
When I clic the String write a good path, but open() don't want this path :(

1. String openPathApp = ("") ;
   boolean openApp ;
     
   void setup() {
     size(300,300 ) ;
   
   }
   void draw() {
     background(0) ;
    
     if(openApp && openPathApp != ("")) {
       println(openPathApp) ;
       open(openPathApp) ;
       openApp = false ;
       openPathApp = ("") ;
     }
   }
   
   void mousePressed()
   {
     //select Input
     selectInput("Select an app to process:", "appSelected");
   }
   
   
   void appSelected(File selection) {
     if (selection == null) {
       println("Window was closed or the user hit cancel.");
     } else {
       openPathApp = selection.getAbsolutePath() ;
       openApp = true ;
    }
   }
   

Your new sketch worked for me in the PDE, but when I exported it as an App, the selectInput would not let me select an .app file - it treated it as a directory and showed me the contents.  I think this is an issue worth reporting on GitHub, but I imagine it's a mac-only problem.

ok, same for me finally i make a new app for the target, that's work in the PDE but like you with the App. I go to make a new report on Github. And hope Ben Fry don't say to me again is not a bug and close my Issue for the third time :)

Ben Fry give the solution for processing 2b9 and less.
For in formation make the changement in the parent app, not in the target app :
https://github.com/processing/processing/issues/1959

The fix described by Ben Fry in the github issue applies to all versions up to and including 2.0.1.  I have tested his changes in the github repository and confirmed that the problem will be fixed in the next release.