The main problem with your code is that you are trying to use a real-valued variable as an index, but indices have to be 1,2,3 ....
Here is some code that stays true to your looping structure:
Vth=255.2;
R2=.332;
Rth=0.59;
Xth=1.106;
X2=0.464;
ws = 1800*((2*pi)/60);
a = 1/ws;
kRange = 1:-0.01:0.0001;
numberOfK = numel(kRange);
for nk= 1:numberOfK
s(nk) = kRange(nk);
b = (3*Vth.^2*(R2/s(nk)));
c = ((Rth+(R2/s(nk))).^2+(Xth+X2).^2);
Tind(nk) = a*(b/c);
end
figure
plot(Tind,s)
Even better, though, is to take advantage of the fact that MATLAB is vectorized:
Vth=255.2;
R2=.332;
Rth=0.59;
Xth=1.106;
X2=0.464;
ws = 1800*((2*pi)/60);
a = 1/ws;
s = 1:-0.01:0.0001;
b = (3*Vth.^2*(R2./s));
c = ((Rth+(R2./s)).^2+(Xth+X2).^2);
Tind = a*(b./c);
figure
plot(Tind,s)
