I experienced the same thing... how do I deal with this
Try copying a similar file on that "Audio/BGM/" folder, maybe copy and rename "kanna-08.whateverformat" to "kanna-09". It's a sort of issue I run into a lot when playing MTL games (as MTL games have to be unpacked, modified and then repacked, a process that makes missing/renamed files a very common issue, especially if links to game files get translated as well), but kinda weird for a non-MTL one.
If that folder does not exist, you'll need to use an RPG Maker Extractor to extract the main game file (somethingsomething.RSS3) and then create it.
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Also, running the game in Japanese locale also fixes a lot of the "missing files" error on games, even those that have been "fully translated". Make sure to always run RPG Maker games with japanese locale, or change your Windows settings to Japan (although that also defaults a lot of software/games to japanese language when you first open). Using this software makes everything easy:
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But again, this developer is known for making spaghetti code that rivals YandereDev... But at least he knows how to make interesting art, scenarios and story. And he releases full games, lol.