Is there a stat for number of missed “open look” shots in an NBA basketball game?
Not for a specific game that I was able to find, but NBA.com appears to have this statistic over a period of time.
From NBA.com/Stats -> Player Stats -> Player Tracking Shots, you'll find yourself on the "League Player Tracking Shots" page.
From here, under "Closest Defender" or "Closest Defender (+10 FT)," you can choose to filter statistics based on your definition of "open look" (which I'm assuming would fall under "Open" or "Wide Open"). From here, FGA minus FGM will give you "missed 'open look' shots" per game.
For example, Stephen Curry missed 4 "Open" shots per game during the 2015-2016 NBA...