I download a bunch of maps here and when i play with the map, my opponent is the computer hard army, it seem like my opponent don't build anything , there is no challenge at all.
Please help. Thank you

That would be to that the maker didn't add the necessary stuff to the map to make the AI work. So the maps where the AI doesn't work are only intended for online play.

Thank you very much , that answer my question.

You could always modify a multi-player map for personal skirmish use. If you're not sure how, look at this tutorial .