Im not sure why but the elytra textures are missing. Normal elytra textures are fine. Im playing with a modpack but idk what would have caused this.

If you mean elytra textures used by powers that use the elytra_flight power type, then it's likely that the client doesn't have the asset(s) loaded. The custom elytra textures have to be put in a resourcepack and be enabled in order to be loaded, otherwise, it will be just a missing texture

ah, ok, thank you. how would i do that? its not just on my client, but for everyone in the server its missing textures as well.

Very late reply, but you can follow the steps presented in this tutorial to setup a server resource pack