Starting ti damping off

Simply buy a smart start germination set online

Also, why are you planting in so little coco. If you don’t have the setup to grow in pellets in a full ropagation chamber just fill a solo cup up with coco, wet it and plant the seed in there. Is that a soap dish? The roots don’t have enough space to grow down hence they’re dying. Go to growweedeasy.com

Humidity a little high at night. Also try putting a fan in there to get a breeze going. Don’t aim the fan directly at the seedling though. Aim it at the wall of the tent so it bounces onto the seedling. That will provide your seedling with a stimulus and release a hormone that will strengthen the stem

Your substrate looks really loose, I've never seen anybody plant direct in coco. Normally the seedling is put in a pellet once it germinates so that the taproot has something to anchor on to. I also noticed in your diary that you are using diluted hydrogen peroxide for germination, unless they are old seeds (over 12 months) I'm not aware of any benefit to this. If they are fresh seeds just germinate them in tap water, hydrogen peroxide is normally used to soften the casing of an old seed which may or may not be viable and you have nothing really to loose. I'm not expert by any measure but I hope this helps.

Is this a photo or an auto?

It does not look like damping off to me. Hard to tell with the picture. But she does not look happy. It almost looks like wind burn. Is that fan in the background osculating? At that stage you don't need much air circulation.