The mod can't add a "races" variable, because that variable already exists in the script, so either they conflict (and the mod is not applied) or the one from the mod is overwritten.
Btw, can you define a variable before the "extends Node" statement?
I suggest to remove line 2 and 19 from the mod and insert the code after line 359 of races.gd, so that the new two races are pushed at the end of the existing list (the mod should start with <AddTo 358> or something like that, the number may be slightly wrong)
Also what Kyler2 said.