Sending supplies by sea posed to North Africa posed a problem for the allies because:
In order to get to North Africa, the Allies had to go through the Mediterranean sea which was mostly occupied by the Germans on the European side.
From these occupied areas, the Germans could have sent U-Boats or submarines to attack Allied ships and stop them from delivering supplies.
Find out more on German U-Boats at https://brainly.com/question/2732308.