What does the line above "Command fail. Return code -8" actually says? It regularly gives some idea what it didn't like. But either
a) the IP you put in is not a valid one
b) a software bug
c) command option menu error (I've seen a long time ago that the option actually doesn't exist when it showed after hitting '?'.
In either case you need to update fortiOS much more modern one. 5.0 is three generations older than the last major version for 800C, which is 5.6.