Why is this narrowing happening

Seeds need 2xstruggle coming up. It's how theu build a root zone This fizzled out Still save Repot..

This might also might happen because insects love those new roots and eat them up so the plant starts dying off from the root up. This is why we always starts in pots indoors then move them outside once they are strong enough. Unfortunately if this is the case there no hope for her. good luck .

You did not plant the seed deep enough.

tbh to me it looks like mechanical damage, like smth crushed the stem

anything from frost/cold temps, over/underwatering to sun burn. plant is dying, cover that part with soil, maybe you will save it. in my first years i was geting it from overwatering and root strangle without oxygen, this year i got during -5 celcius nights.