I had the same Issues, try moving both the Runemate.jar and Runemate.sh file so the user folder (its icon looks like a house)
I placed it there and ran the Runemate.sh
Hopefully that helps!
I Had the exact same issue, it probably depends on where your runemate.jar and runemate.sh file are saved.
I reinstalled Java 8 JDK (cause I was getting fed up) but the real fix for me was putting both files in my main user folder (Which should be your computer account name)
I was still running...
