How to short all the internal nets to VSS with skill

Nets don't have an attribute segments so this will do nothing. Equally, your call to schCreateWireLabel has the wrong number of arguments, so even if you changed net~>segments to net~>figs then it would fail because of that. 

Finally, you may also need to run schCheck() to extract the connectivity in the schematic if you are creating wires using SKILL. So something like this would work instead:

lib = "TEST"
cell = "RES"
cv = dbOpenCellViewByType(lib cell "schematic" "schematic" "a") ; "a" for append mode

schCreateWire(cv "draw" "full" list(0.0:0.0 0.25:0.25) 0.03 0.03 0.0)
schCheck(cv)

foreach(net cv~>nets
  when(net~>figs
    schCreateWireLabel(cv car(net~>figs) centerBox(car(net~>figs)~>bBox) "VSS" "centerCenter" "R0" "stick" 0.0625 nil)
  )
)

Andrew