Hello! Sorry you’re having issues installing the DLC.
1) Are you using mac or pc? (so we can help you better)
2) The base game on Steam will not work with the DLC file here. The base game and dlc are uploaded differently on steam than here on itchio.
3) For the regular epilogues, you need to have completed both the romantic and dramatic ending in order to unlock the epilogue for that route. It won’t be unlocked if you only complete the route once.