Shoot, another instance of too big a pot for the size of plant. Too wet. Roots are rotting and not happy thus the top growth is dying. Sorry sweetie. You are going to have to try again, yes? See some of the other question/answers about avocado germination and care. A vastly smaller pot, 4" would be a start. Only potting soil, no rocks or gravel below the soil in the pot above the drainage hole. Just soil. Do not water when the soil is moist for the upper 1" and we are talking about the 4" size pot, not your 12" in your picture. (is it 12"???)
Purchase bottled distilled water, do not use tap water. I can see signs of high salts on your clay pot. Clay is great! Tap water is not.