A build check warning in the area "client/index.html rules". A warning: the game still publishes, but fix it when you can.
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
The client is one page that talks to the room only through the kit (Playable.connect). It may load no other scripts or URLs, and it must render with the presentation mode the manifest names.
How to fix it
Load the kit's script and nothing else from outside, call Playable.connect, keep everything else inline or in client/assets, and never navigate the page.
See also: every error code, the API reference and the docs.