Question

why can't the enthalpy of magnesium be measured using a coffee cup calorimeter?

Answer 1

I assume you mean Mg to MgO. How will you burn Mg under water?

Answer 2

No, the question only says Mg

Answer 3

Then I have no idea what the question means. The delta Hformation Mg is zero. For anything else with Mg there must be some kind of reaction.