But get the light 5" higher to test it, and just in case only veg switch on at this stage.
To me it looks more like to much nutes as bleaching should be on the top leaf.
That affected leaf is the first to die anyway but should take some more week to do so.
Don't look like overwatering.
More info would help.
What strain ?
Medium ?
Any feeding ?
How old ?
Temps and humidity ?
if it was light you would see them curling and i dont see the leaves curling or any kind of heat stress related that normally you see on the top leaves.
Could be Nute burn, which is common with younglings.
What kind of drainage do you have and what kind of feeding schedule? when you stick your finger in the medium is it soaking wet? if so let it dry up before you feed again.
There are alot of variables at play so we have to do a step by step walkthrough to help.
oh i guess the links i sent are blocked... well if you google grow weed easy which is the name of the place or plant deficiency problem solver you will find some good step by step problem solvers with pictures to help.
Distilled water, no nutes, soil is some guano bag stuff from the grow store. Stoner old guy there said it would be good for my young plant. I have some boutique seeds I'm going to sprout as soon as my box is fixed up.
12“ is the closest you can get a P450 to the tops, and the burn will be bleaching, so very noticeable, and it will crisp up within a day on the burned area - that looks like over watering to me
I would move that LED up 24"-30" from top of plant. LED/HPS/MH really close when there that young can shock & stunt growth especially when moving from cfl/t5-t8, move that light up until the plant becomes acclimated and then lower.