GrowthRequirement.maxBrightness is exclusive and has a default value of 16, but a max brightness of 16 in a MineTweaker script is not allowed.

Fix: Change this line to less-than-or-equals (<=) instead of less-than (<).

(Also, the error messages here and here should be swapped.)

Light goes from 0 to 15, 15 included, so having 16 as an exlcuded upper limit is fine.
http://minecraft.gamepedia.com/Light#Brightness

Error message have already been fixed on my other branch, but thanks for notifying anyway.

Okay derp I see your point, consider it fixed