So after reading this, I got curious. I have a spellcaster Sim who I cheated basically as many special traits onto as I could think of. I didn't think of the werewolf or the alien ones since she was a spellcaster. So I got on her, made her human, and was able to plant the seedling. I then added the werewolf trait. It made her a werewolf, so I had to remove the actual werewolf but left the trait. She was still able to. When I tried to add the alien ancestry one, it wouldn't let me plant. So it has to do with the alien hybrid trait (trait_OccultAlienPart). After removing the trait, it allowed me to plant it again.
And like others said, you don't place the seedling on the ground and click it; you have it in your inventory and click the ground. Hope this helps! :)