Sorry you are running into this with your games. There are a few things to look into, if you haven't already.
- The UnityPlayer.dll should be*right* beside the morticianstale.exe file. DLL files can get confused sometimes.
- Sometimes a program gets blocked by anti-virus or anti-malware. Try briefly turning off any live scanning.
- We've heard of people running into issues where enterprise software can muck things up. Look at what other applications are running, including in your task tray (or use task manager) and close any that you don't need running at the moment.
- Installing to another folder sometimes helps, though because it did run once, I don't think that is the case here. That said, the itch.io app is quite nice and has sometimes helped with these kinds of issues by managing downloads well.
If none of those work, feel free to let us know and include a screenshot of the error. Since the issue seems to likely be something conflicting with Unity games in general, I can only help so much but I'll do the best I can!