Like many other players, I bought this game, however it wouldn't start. I did some snooping around files, and it seems the devs uploaded an incomplete or dev build. Luckily, I found the issue and have come up with a fix.

1. Locate a "steam_api64.dll". I won't suggest downloading it somewhere online, because when I went searching, everything looked very sketchy. Instead, a lot of other games already have it in their download folders. For example, I found it in the root folder of my Grand Theft Auto V install. Just find it and copy it to your clipboard.
   
2. Navigate to your OnlyFap install. This can be done by right clicking the game in your Library, hovering over "Manage", and clicking "Browse Local Files"
   
3. Navigate into the folder called "OnlyFap_Data" in the OnlyFap install folder.
   
4. Create a folder called "MonoBleedingEdge" (without the quotes), inside the "OnlyFap_Data" folder.
   
5. Paste "steam_api64.dll" into the "MonoBleedingEdge" folder.
   

Once you have done all that, you should be able to start the game!

I will note that you start off with like $900 quintillion, and there are a few parts that aren't localized correctly, so it seems rather apparent to me that the game is either a release build or they uploaded the wrong files. Either way, I just wanted to help out anyone who wanted to get the game running asap and didn't want to wait for the devs to release a patch.