Are you using FSX or P3D V5 ? If yes, it's entirely normal that custom textures cannot be created in FSX or P3D V5, since they require Render to Texture with Direct X 12, which is only supported in P3D V4.
Which is why, we added a lot of new operators since P3D V5 came out so, you can just search if the one you need is there. If it's not, please report it and indicate where it operates.
