Worng Fertilizer

Not sure what your growing in., you don't have a diary stating your setup. What lights are used and how far away? What's the temperature? What PH is your water? It's not always nutrients causing problems. I've seen many first time growers using soil mediums that require nutrients from the start. Myself, I wouldn't want that challenge without experience but you have to learn somehow. The easiest time to kill a plant is seedling to early veg stages. There could be several reasons this is happening. Most common is overwatering. With my plants I do a thorough PH'd watering of the soil prior to putting germinated seed in. I don't do anything until it is above the soil. Then I only mist the soil around the seedling probably soaking down an inch or two. That's done every other day until the first leaves are out, those single finger ones. Just my way of doing it. At the stage you are now in my grows I would start watering the whole pot. It could be 7-10 days before the next time but that depends on the medium.

Maybe also Nuts lockout, I would flush it & start gentle to add the needed nuttiens step by step

It looks more like a deficency instead of a burn…