NS3 programs points to files starting from your ns3 root directory. In your case: "
~/Desktop/NS3/ns3-allinone/ns-3.26/"
I usually simply place my TCL file in "~/Desktop/NS3/ns3-allinone/ns-3.26/" or create a directory under it for all my traces and logs.
I do notice that you have an error in your path.
You're working directory "~/Desktop/NS3/ns3-allinone/ns-3.26/" , so maybe your mobility trace file is in "~/Desktop/NS3/ns3-allinone/ns-3.26/scratch" but you're passing a different path, /ns3/Desktop/NS3/ns3-allinone/ns-3.26/ , which does not have your TCL, or might not even exist. You're also using $Home, but you probably mean $HOME. I'd usually just simply use ~ as this means home directory.
For simplicity, place the mobility.tcl under ~/Desktop/NS3/ns3-allinone/ns-3.26/ and simply use --traceFile=mobility.tcl and that should work. or simply leave it where it is, and use --traceFile=scratch/mobility.tcl
Hope this helps!