Processing (PDE) ant build Error

Hey I need some help.
I want to change some code in the Processing (PDE).

That’s why I downloaded the Github library.
Then I copied the “app” folder to the desktop and opened it in eclipse. There were a few bugs as stated on the website. so I followed the instructions:
You’ll have several errors, because you need to build the projects once with ant (on the command line: cd / path / to / processing / build && ant) so that the “generated” folder is created. After doing that, select the processing app project in Eclipse and hit F5 to refresh it.

but every time I want to build it with ant (in the console), this error comes up:
C: \ Users \ chaos \ Desktop \ app \ build.xml: 13: Please build the core library first and make sure it is located at …/core/library/core.jar

Does anyone have an idea of what I can do?