Using a schematic in singleplayer

This is the single most common misunderstanding about schematics, and it is not your mistake — the two files arrive the same way and nothing explains the difference. A world is a folder with terrain, a spawn point and player data. A schematic is one file holding one structure and nothing around it. Putting it in the saves folder does nothing at all, because there is no world in it to open.

1. 1Install a mod loader — Fabric or Forge — for the version you want to build in.
2. 2Install the WorldEdit mod for that loader. It is the same tool as the server plugin, in mod form.
3. 3Put the .schem file in the schematics folder inside your Minecraft directory. WorldEdit creates that folder the first time it runs.
4. 4Open the world you want to build in, run //schem load with the file name, stand where you want it and run //paste.
5. 5Use //undo if the position is wrong. It works as reliably in singleplayer as it does on a server.

There is a mod-free route, but it is more work: open the world to LAN, or run a local server with the WorldEdit plugin and connect to it. Some people prefer that to installing a loader. There is no way to use a schematic in pure vanilla singleplayer with nothing added — the format is not something the game itself reads.