When in your terminal type the command: su
if it asks for a password type in your login password and enter. then the # symbol should show at the begin of your line.
Or you could install "sudo" by using your app manager or type the line "apt-get -f install sudo" in your terminal and it should install.
Then you can type: sudo flasher -c -F ImageFile.bin -f -R or sudo flasher -c -F /Path/To/ImageFile.bin -f -R.
If nothing els works you may download a differant image from navifirm like the MEA4 that i'm using,because your image might be corupt. And try to flash it with the new Image. I recommend you copy the download link from navifirm to clipboard and use a different app to download the image like "wget in your terminal. e.g: wget /link/from/navifirm. Or you can download it with your browser by pasting the link there. I hope this helps