For me, it's not really that I can't do it, it's more like I can't do it yet.
I've got one that's an Eros and Psyche retelling, but the only thing stopping me from doing it is the fact I have no idea what being an adult is like (holding down a job, going to bars, getting drunk etc.) which is fairly integral to the setting. So RIP.
Another is a story set in college - except I'm not in college yet, so I have no idea how it works. Also I'm afraid that people are going to think I'm ripping off Rainbow Rowell's Fangirl because they're both about fanfiction writers going through college... When I'm not.
I could make it so they're both set in high school, but one thing I've learned recently - I loathe writing about teen drama. I think they're usually super contrived, and I am a teen.
I think the only idea I have that qualifies for "I can't write it ever or at least for a really, really long time" is a fantasy story about a plague - and considering the current events... you know.