A build check error in the area "Package files". An error: the game cannot be published until it is fixed.
The build check runs on every game LobbyLab builds, and each problem it finds has a stable code, the exact message, the file and line when it names one, and a link to this page.
What the check looks at
A game package is manifest.json, server.ts and client/index.html, with optional client/assets/. The check first makes sure these files exist, are plain files and fit their size limits (manifest 64 KB, server.ts 192 KB, index.html 512 KB).
How to fix it
Keep exactly these files at the package root, as plain files (no links), within their limits.
See also: every error code, the API reference and the docs.