I can’t remember if you are indoors or out but it could be a lot of things. Abrasion damage, bugs, nutritional (nutritional seems like a long shot given how young it is unless maybe the soil was super hot to start). On the plus side it is just a cotyledon leaf (the first set of smooth leaves you see) so they were going to very shortly die and fall off anyways. They basically just bootstrap the plant out of the seed and get it started, until the main leaves start to grow. Actually, it is possibly just the first stage of those leaves dying. Keep an eye on things but don’t panic yet.