Since version 1.15.1 is an older legacy version (released around 2014-2015), you must ensure the mod specifically supports or 1.15.x .
Finding specific content for requires looking back at legacy mods, as the game is currently on version 1.58. In version 1.15, bus mods were third-party additions that replaced specific truck models or appeared in dedicated "mod dealers". Popular Bus Mods for Version 1.15.x