Fairy Seedling Won't Plant

I was having the same problem and then I clicked on the ground. You don't drag the seed or click the "plant" option. You click where you want to plant it and then there's a new option to "plant fairy seedling." It's its own thing.  

I also just thought this was a seed for an actual plant.  But it's how you summon mother nature to become a fairy. I have no idea where I got it as I didn't do a quest to become a fairy and don't plan to be one. Luckily you can just cancel out. 

Spray some grass on your lot and then try and click on it to plant. I used the new grass texture. The planting option is its own click option. 

Is your Sim currently an occult or a half-alien ? You can only plant it if you're Sim is a human (and possibly a ghost)

My SuperSim I was trying to make a fairy cannot plant the seed. She is not actively any occult, but she was born with these traits: Grim Descendant, Greater Wolf Blood, Ancient Bloodline, Sulani Mana, Father Winter's Baby, the 'secret alien handshake', & was conceived in the lighthouse. The alien in their family tree is something like a great-great grandfather level deep... Could that handshake part be stopping her from being able to plant the seed/become a fairy? 😔 She could ask about becoming a fairy, and she could ask a fairy for a seed or purchase it on the computer, but neither of these seeds would let her plant it. 

If they do any testing on this and want to reproduce the issue, I do have this sim(s) (actually, I tried with 4 of them with the same result, and they are all in this household in the gallery)... https://www.ea.com/games/the-sims/the-sims-4/pc/gallery/450EEF5BC5AB489C94A221480DFD690B

I was mainly trying with Emilia and Ellie. I also tried with Justin and Briella (Briella has all the same 'born with' traits, except not Grim Descendant).

Edit: I just tried moving one of my previous SuperSims that I kept though she did not have the alien handshake and Greater Wolf Blood. She was able to plant her fairy seed. She is a Father Winter's Baby, has Sulani Mana, and Ancient Bloodline. So in her case I don't know whether or not the Greater Wolf Blood trait being missing made any difference and if it was only the Alien handshake being missing that mattered, but at least we know the Father Winter's Baby trait didn't block it.

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! :)

I saw another post only humans, fairies and ghosts (as long as fairy or human I guess?) can plant the seedling.

Yes. It is non-occult or fairy only. The original poster had a "non-occult" with some Occult heritage traits from parents/ancestors. The alien hybrid trait from having an alien in their family history appears to be the culprit, as it apparently still counts as being an Occult.

Yup... it's weird. I have 'SuperSims' that I mentioned before with all of the previous 'occult' possible born-with traits and they also had the alien handshake (details on that in my previous comment). So... it must have to to do with an active occult, and that the alien trait must always be active. However, I can have the sim actively any other occult while having the alien handshake trait. This fairy trait is the exception. So I went back through the 'SuperSim' 'breeding' process, but this time all I left out was the alien ancestor (many generations back) so the infant does not have the alien handshake when she gets to teen or older.  So, she has the 'born with' traits of Sulani Mana, Ancient Bloodline, Greater Wolf Blood, Father Winter's Baby, & was Conceived in the Lighthouse.  I tested her by aging her up and she is able to plant the fairy seed, be a fairy, and even have an offspring with the High Fae Lineage trait if the father is a fairy. However, if she has this baby with a fairy father the baby only inherits Sulani Mana,& Strong Bloodline from mom.
https://www.ea.com/games/the-sims/the-sims-4/pc/gallery/25B4F64A98454AEFB055979F6A914350