I made and map for skirmish
and the enemy only build structures on medium
but when i play on hard they only build some dozers but no buildings
how can i fix that ?

Did you add the players to the map using the "Add Skirmish Players" button?
Did you NOT add any scripts to skirmish player folders?

If these both are true then default scripts will load into the map.

Adding areas and pathways  to your map will allow the AI to utilize their scripts.