Minecraft skin missing texure
Solution 1:
From the link provided, it seems like the hair is part of the second layer of the skin, and the option that controls the visibility of this layer has been toggled off. To show the second layer of the head, navigate to Options > Skin Customization > Hat: On.
For reference, you can learn more about skins by going to the Skin page in the official Minecraft Wiki here, as well as the Skin Customization menu here.