Can barley flour be used as a substitute for barley malt extract?
No, they are quite different. Barley flour is just the milled grain, but malt has been sprouted so it has much higher enzymatic activity and is much sweeter. It is usually used to precipitate an enzymatic reaction in your bread, which plain barley flour won't do. The recipe may work by just leaving the malt extract out, but it may brown less or rise more slowly.