Buak disables building on vehicles in a config override file inside the workshop folder that will overrule your (server) settings. It is possible to re-enable building on vehicles for single player and if you're hosting a private server but that involves opening the workshop content folder in your Steam workshop downloads and changing the config file so it does not overwrite your settings (or make it overwrite settings with build on vehicles = true). I don't remember the file path for that config file though. If you are connecting to someone else's server though I don't think there is anyway to change that, unless the server host agrees to change the config override file in the workshop folder. Also if Bauk updates it might change the config override back to no building on vehicles, in which case you'd have to find and change the config override file again.