Can I melt an aluminium can with magnesium ribbon?

You must know: Heat rises. Little of the heat from the burning of the Mg ribbon goes down into the can. It all goes up into the air. Perhaps what you should try is to ignite a reasonable quantity of the Mg ribbon and as soon as it ignites, place the concave bottom of the aluminium can over the flaming Mg. The heat generated will go directly into the can and ( hopefully) melt it. There must be close contact between the burning Mg and the bottom of the can. Try that.