HELP; I messed up probably?

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Over watered and possibly a soil issue causing the same thing. Poke some holes on the cup. If you went and bought an outdoors potting mix it's gonna be too heavy for seed starting because it does allow air so the plant drowns. I'm not seeing perlite but I do see peat and a tiny bit of coconut with a lot of bark. You got some wet soil that doesn't breathe. Or I'm an idiot so lesson learned and always do a little research. Don't just rely on others here, trust me on that. Master means you fished a lot of likes in a year or maybe you've been growing 20 years with under a hundred stars so you are a newbie. Although well intentioned take all advice with a grain of salt.

Is it soil thats loaded with fertilizers? If so you are burning the seedling

I am gonna go with too wet and possibly wrong substrate ph.

Some lockout happening why without how, in what and were you grow is hard to tell. If it's soil - 100000% lockout, seed has enough food for up to two weeks in theory ... If it's Coco , they start feed earlier and could be calcium deficiency starting .. Bad pH, too hot air and intense light with overwatering are main mistakes. All the best !

Pretty typical seedling, nothing much to worry about, give it a few days to get established and I am sure you will then be wondering why you were asking this question.

Need some more info mate but for a start it could do with some new soil thats a bit more friable and then a light feeding