Mashup Pack Texture not Working

I bought the How To Train Your Dragon mash up pack a few years ago, and it worked fine. Today, I was playing it on my Nintendo Switch Lite, and it worked perfectly, but when I tried to make a new world of it on my computer, the textures were missing! I tried copying a previous version of the world, but when I opened it, it said the texture pack was missing! I am really upset, especially because I just spent a little under an hour making a skin to roleplay in that world.