Hi i found out what was wrong!!
I don't know if it's the daw installer or anything else, but a file was missing in C://programs/steinberg/cubase10.5/components. It's a file named FaderPort.dll or faderport_cubase.dll . It was at the wrong place. I found it in C://programs/steinberg/sharedcomponents. You just have to copy it and paste it in the good folder and voilà! The faderport classic appears in the device list and works perfectly!