HI , to create a face all the edges have to be joined , co planer and without any edges protruding into the proposed face. Without having the file its hard to tell what your problem is. The triangles suggest that the edges are not co-planer . i would start again with the garden and erase the lines already draw, If you go to window>style>edit tab and select by axis with the drop down window at the bottom any off axis lines will show up black.
Phil