Skins not working in the World, Realm, and Servers

It all started after I downloaded a free world pack which was the one with the dinosaurs. After the day, I downloaded it I went to the servers to go play with my cousins, then I changed to one of the skins that was in that pack and when I went to go change my regular skins that I import it said that it's not a skin. So I was kinda confused, I tried many of the skins that I download from websites and the app skinseed but it doesn't work and it shows invisible on the screen when I play a world and it shows a yellow Steve when I joined the servers. It has been past few days now since it happens and it still doesn't work. Please help me fix it.