I'm not aware exactly when this started becoming an issue but it happened sometime around when the 1.16 snapshots came out. I saved a structure on a different world and tried to copy the same file to a different world but it says "structure is not available" I tried a different world, it still would say "structure not available" it worked prior to the 1.16 snapshots. I'm not sure if this is a technical issue or a game issue but it doesn't work anymore.