Hi,
for ka = 0.17 and landa = 0.13 there is no extrema. You can plot the derivative to see that:
syms x
ka= 0.17
landa=0.13
y = (ka/landa)*((x/landa)^(ka-1))* (2^(-(x/landa)^ka));
z = diff(y)
steps = -1:0.1:1;
g = matlabFunction(z)
plot(steps,g(steps))
0 is a asymptote here. So you will never reach 0. Thats why you get this message.