You also need to specify the time value in plot(). Try this
num = [0.1 0.35 0];
den = [1 3 2];
sys = tf(num,den);
figure();
step(sys);
[y, t] = step(sys);
grid
title('Respond of sytem Subjected to initial Conditoin')
xlabel('t')
ylabel('Output x(t)')
hold on;
[max1, idx1] = max(y);
[min1, idx2] = min(y);
plot(t(idx1), max1,'o')
plot(t(idx2), min1,'o')
legend('y(x)', sprintf('Maximum y = %0.4f',max1),sprintf('Minumum y = %0.3f',min1))
