The game is already in Malbec
Malbec already has a copy. Play it from there.
What happened
The game you are bringing over is already installed in Malbec’s copy of its store, so Malbec didn’t copy it a second time.
The fix
Click Show in Finder to see that copy, or press Play.
When to write to us
If Malbec’s copy doesn’t start, write to us with a diagnostics file.
Email support@malbec.app. For a diagnostics file, choose Help › Export Diagnostics… in Malbec and attach the zip it saves. It has no account details, and your home folder is masked.