Firstly, I noticed that you have commented out the fahrzeugErstellen() function in your server.lua file, which creates the vehicle with the specified model ID. Therefore, when the fahrzeugErstellen(playerSource) function is called, it is not creating a new vehicle. You can uncomment the function to fix this issue.
Secondly, I can see that in your client.lua file, you have commented out the conditionals for checking whether the texture and model files exist. This may lead to errors if the files do not exist.
Additionally, in the fahrzeugModellieren() function, you are requesting the model ID using engineRequestModel('vehicle', 544), but you should use engineLoadDFF('pdBus.dff') instead. You can then use the returned model ID to import the texture and replace the model.
Lastly, in your onPlayerJoin function, you are passing commandName as a parameter, but you are not using it. You can remove it to prevent any potential issues.