cant play multiplayer with friend

cant play with a friend , game says to copy stg file manuly,dont know what this means pls hlep

Focus Team
  1. Right click on 'Spintires: MudRunner' and click 'Properties'.
  2. Click the tab 'LOCAL FILES' then click the button 'BROWSE LOCAL FILES...'.
  3. Delete the file '' and close the window.
  4. Click the button 'VERIFY INTEGRITY OF GAME FILES...'.
  5. Steam will redownload the file ''.
  6. Once done, run the game and test.

Is this media file modable? I noticed it is just packed with zip so can I for example change sound file and pack it on normal compression and game will see this new file or will there be problems?

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