Stress is defined as force applied on area.
Mathematically,
Stress = Force / Area
Dimensional Formula of Force= M1L1T-2 [Check how Dimensional Formula of force is derived]
Dimensional Formula of Area= M0L2T0[Check how Dimensional Formula of area is derived]
Putting these values in above equation we get,
Dimensional Formula of Stress = (M1L1T-2)/(M0L2T0)
Dimensional Formula of Stress = M1L-1T-2
SI unit of Stress is Nm-2