You skipped a version (e.g., RX11->RX13 without RX12). Fix: You cannot fix this with a patch. You must perform a clean install of the full repack (backup UserData, delete the game, install the latest full repack).
Update BepInEx to the latest version and check for "Data Extender" updates if error messages appear. koikatsu partyrepack upd
This is where 90% of users fail.
: Place the single .exe file and all associated numbered .bin files together in the exact same temporary folder on your desktop (not inside the game directory). You skipped a version (e