PS : I answered below to your error, before seing that this error is in fact old of 4 monthes. You certainly fixed it, but as I spent some time to write this answer, I keep it, perhaps it will help someone else in the future...
@SupSuper : modular ruleset are a must, it's a pleasure to modify them or create sub rulesets !
As I met same error message, and you are apparently working on terror missions, I had a look at your ruleset and I saw some errors
Check at lines 3824-3826 : you defined only one itemSet for alienRank 6
You have to define one itemSet for each of three technology advancement levels
Also replace
itemSets:
-
[]
with
itemSets:
-
[]
-
[]
-
[]
Same at lines 3814-3818
itemSets:
-
- STR_PLASMA_PISTOL
- STR_PLASMA_PISTOL_CLIP
- STR_ALIEN_GRENADE
to be replaced with
itemSets:
-
- STR_PLASMA_PISTOL
- STR_PLASMA_PISTOL_CLIP
- STR_ALIEN_GRENADE
-
- STR_PLASMA_PISTOL
- STR_PLASMA_PISTOL_CLIP
- STR_ALIEN_GRENADE
-
- STR_PLASMA_PISTOL
- STR_PLASMA_PISTOL_CLIP
- STR_ALIEN_GRENADE
You should make same correction for TerrorShip
And if you are intended to test other UFOs, I can see following are wrong too
3856
3864
3877
3887