Save: https://drive.google.com/file/d/1zuqKDplAjA52g6HKA-YKKkGadoz_wcHW/view?usp=sharing
Civilizations with the ability "Exterminator" are supposed to be able to colonize from the start of the game the planets of type "Toxic".
Koraths ("Exterminator" ability) cannot colonize this type of planets from the start of the game.
Edit: This is indicated in this changelog (v3.5).
"Exterminators can now colonize toxic worlds"