gabi
In your terminal window
If using windows type
echo %CD%
If using linux type
echo $pwd
That will print your current directory.
If you are not inside the directory that the fastboot program is in, move into it, then your fastboot cmd will work
However, u can't install grapehene on any device, only pixel phones to my knowledge...
So give up with your laptop and put a Linux distro on it like ubuntu or Debian etc