Lacking While It's Cracking is a slang term which means that someone is not ready (Lacking) when things are going down (Cracking). For example, if a gang member is walking through a rival gang turf without a weapon (Lacking), and a shootout breaks out (Cracking), this individual is lacking while it's cracking.
This term has been used by numerous rappers, especially drill rappers from Chicago such as Lil Herb and Lil Durk. In Lil Herb's 4 Minutes of Hell Part 2, he says "I ain't lackin' while it's crackin' Something long on my hip, big pole n*gga."