When installing X-Plane via digital download, at times the installer will not be able to access a file. It will wait, and retry it, but eventually it may skip that file instead of canceling the entire installation. It's possible that's what happened in this case.
If you start the installer and pick "add/remove scenery", it may not even realize it is missing--the whole world may still be colored in as if everything is installed.
If you know what general area it's missing from, I would recommend you uninstall a few of the tiles around it, and then re-install them. Re-installing a handful of tiles should only take a few minutes.