What's the problem with my plant?

It can be 3 things for what i know. 1. The sun is burning your seedling. 2. If there is not enough moisture in your soil then the nutrients gets more concentrated the more your plant is trying to suck up the water. So this would be nutrient burn. 3. The soil can be too hot for the roots.

Acqua

I agree with the other answers, and I would add to keep her away from the mid afternoon sun (looks to me you are doing outdoor), just morning and evening! she is too young for the powerful afternoon sun

She looks dry , but the burning is likely the substrate is too rich with nutrition is what it looks like.

She looks pretty dry to me, mate.... are you watering her at all? Don't water TOO much or you'll end up drowning her but she needs SOME moisture to do her thing! I would add some mycorrhizae to help the roots out - at least some microbes in your water which will also do wonders for the root system. No nutes, though! Still much too young for those! Good luck!