Non standard disk brake post mount
Mountain bike forks will normally direct mount to fit a 160mm rotor. The 180mm adapter you purchased will add 20mm to rotor size - so should (might) work with a 160mm rotor. You would need an adapter to increase 40mm rotor size ir you want to go to a 180mm rotor. 

You could try a 203mm adapter or drop back to a 160mm rotor - either might work but I have not had experience with this setup. 

Looking at the post mount, I would be a little concerned the mounts are up to the forces a 180 rotor could generate and would be checking the specs from the fork manufacturer.