Missing Creations and Skyrim files

The message, what it means, and the steps to fix it. SkyrimScape does not install these for you: the content is Bethesda's and is bought and downloaded through Steam.

What the message says

When you start the game, SkyrimScape checks your Skyrim folder against the Creation Club files this realm uses. There are two kinds of message.

"Your game does not have N of this realm's Creation Club plugins." Items, people and places from those plugins do not appear for you. You can still play; the rest of the world is the same as everyone else's. This is the Anniversary Upgrade's content.
"Your Skyrim folder is missing N file(s) every copy of the game has." Some of the game's own files are damaged or missing. This one is worth fixing first, because it can stop the game from starting properly.

Fix a missing Skyrim file

Close SkyrimScape and Skyrim.
In Steam, open your library, right-click Skyrim Special Edition and select Properties.
Select Installed Files, then select Verify integrity of game files.
Wait until the check finishes. Start SkyrimScape again.

Get the Creation Club content

The content comes with the Anniversary Upgrade. You need it on Steam first; the download only brings what your account owns.

If you do not own the Anniversary Upgrade, buy it from the Skyrim Special Edition page on Steam.
Start Skyrim from Steam, not from SkyrimScape.
On the main menu, select Creations.
Open Options and select Download All Creations in my Library.
Wait until every file is downloaded. Close Skyrim.
Start SkyrimScape. The message should be gone at the next login.

If you do not want the Anniversary Upgrade, you can play without it. The items, people and places from that content stay out of your game, and another player's Anniversary armour and weapons are drawn as the closest standard piece.

If the message stays

Check that the Creations download finished for every file. Run Download All Creations in my Library again; it skips files you already have.
Restart Steam and Skyrim after the download.
If a Skyrim file is still missing after the integrity check, reinstall Skyrim Special Edition from Steam.

When you log in, the chat names the Creation Club plugins your game is missing, so you can check them against your Creations list.