You will get an error if Flash Fill cannot find the pattern. The first thing to check is that you're using Flash Fill close to the source data (e.g., in the adjacent cell). For instance, it won't work ...

I once made money moonlighting as a VisiCalc and Lotus 1-2-3 instructor before Steve Ballmer ever typed “=sum(Profits)” in an Excel 1.0 spreadsheet cell. And a friend and former colleague from my ...