Issue
The multiplayer mod is not found in the BeamNG mod manager.
The Launcher shows an error message as the title says.
Workaround
This has to be done everytime there is a new mod release
Visit the BeamMP from GitHub
Download the latest stable (green) “release” BeamMP.zip and place it in your mods\multiplayer folder.
The mod should now be in the mod manager and the multiplayer button available on the main menu.
Possible solution
Follow this guide for a possible solution to the ERROR in the Launcher.