Confusion between equations of kinetic energy and mass-energy equivalence

The present mass of the Sun is $2.0 times 10^{30} mathrm{~kg} .$ The Sun emits radiation at an average rate of $3.8 times 10^{26} mathrm{Js}^{-1}$. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass. $1y=3.2 times 10^{7} mathrm{~s}$

My incorrect solution:

begin{array}{l}
P=frac{W}{t} Rightarrow t=frac{W}{P}
text { One millionth of the sun’s present mass is } 2 times 10^{24} kg
therefore t=frac{frac{m v^{2}}{2}}{P}
=frac{frac{2 times 10^{24} timesleft(3 times 10^{8}right)^{2}}{2}}{3.8 times 10^{26}}
=2.36 . . times 10^{14} mathrm{~s}
=7.4 times 10^{6} mathrm{years} text { to } 2 mathrm{sf}
end{array}

Why can’t the equation for kinetic energy be used in this context?