I have both Black Mesa and SDK base source 2007 installed. Black Mesa is also in my library. But, every time I go to launch the game, I get an error message with the above title, and it says; “(x86)\steam\steamapps’my name’\sdk base 2007\bin\filesystem_steam.dll” Does anyone know how to fix this? 
Try to verify the cache for Source SDK Base 2006. It needs to be converted to SteamPipe.
Edit: It’s what I’m doing right now. Hopefully, we’ll be able to play the game without anymore problems.
Edit #2: Never mind. Still not working.
Edit #3: I decided to opt in for the Steam client beta and I get the cfileSystem_steam::init() failed: failed to find steam interface error, but after several attempts, the game starts working. I hope Valve fixes this issue very soon.
I got the game to work last night. I’m not sure how I did it, but I did. I figured I’d try uninstalling the sdk sources, but when I tried, nothing happened. Out of no where, Black Mesa just launches, and it does so without a hitch… Really really weird.
Any luck on your end?