。
There was an awning over the sidewalk.
Literal
Sidewalk [gen-の] above [on-には] sun-shade [subject-が] existed.
~の上 is the position noun 'above, on top of' — used after a possessor with の. には on 上 topicalizes the location ('as for above the sidewalk'). 日よけ literally 'sun-avoiding' is a compound noun from 日 + 避ける ('avoid'), meaning awning, sunshade, or anything used to block sunlight.