When i want to use the chroma key simple it doosn´t work and the green background dont go away
sry for my bad english im form Germany
Works fine here.
But you have not given us anywhere enough information about what steps you are taking, which version of Shotcut you are using or even which operating system.
I don’t think anyone here are mind readers 
Basic informations here :
https://shotcut.org/howtos/getting-started/Erste_Schritte_mit_Shotcut.pdf
You can try to search if there is a youtube tutorial in German.