Question

For an enthalpy lab, why would calculated value differ from the theoretical value (in percent error) for the heat reaction of Mg/HCl and MgO/HCl

Answer 1

The usual things like misreading thermometers, invalid calculations, poor lab technique and all of the other errors likely to be made by a student.
Mg may have not been pure, student may not have removed the oxide layer from the Mg ribbon, heat flow into or out of the calorimeter, calorimeter calibration may not have been done correctly, etc.