If you are here then you are playing post forsaken and have boosted a character to lvl 30 skipping the missions to get your other sub classes.DO NOT PANIC
It may seem as if the achievement is locked but it is not.
You will receive a mission from Ikora that will require you to get feathers of light from high leveled enemies. My suggestion is to do public events on the tangled shore because the enemies will drop then quite frequently.
Once you get 200 feathers(do not worry its not as hard as it looks) a mission on IO will appear.
Towards the end of the mission you will receive a seed of light to use on your character. It does not matter which sub class you choose just go to one of them and use the seed. Once you do that subclass will start to become unlocked. Once you kill all the enemies and finish the mission the achievement will unlock once you go back to orbit.
I had this issue happen to my warlock and it unlocked once i finished the mission.
Hope this helps and ends any panics you may of had. Happy Farming!