I prefer to use the block of feta in this recipe because it gives the dish a creamier texture. The block of feta melts better and creates a smoother texture than crumbles, thus is preferred in this recipe. However, you can definitely use feta crumbles if that's what you have on hand. If you can’t find a block of feta cheese, you can use feta crumbles. They will work just as well in regards to flavor for this recipe. Just keep in mind that the flavor will be a little different since crumbles are saltier than blocks of feta, and you may no have that creamy sauce you expect.