Basically it needs an airport elevation (a runway wpt) to construct a VNAV path (normally 3°) from, until then the decent phase won't work as there's nothing to vertically navigate, there is no path and therefore no VNAV.
So trying to descend to 6000 using descend now won't work as it has no idea where the path is and therefore can't decide on VNAV SPD or PTH.
Last edited by CCA; 7th Jun 2016 at 10:44.