Your values of M are not valid indices for u. Your indices must be integer values >=1.
A=1:5;
% This works because the index is a positive integer
A(2)
% This does not work because the index is a decimal number
A(0.2)
The most robust solution would be to create a vector M, and then have your for loop loop through each element of M, with the loop counter being used assign the result to u.
M = .1:.1:5;
for v = 1:length(M)
u(v) = M(v).* sqrt(y.*Ra.*Ta);
disp(u)
end
Even better would be to get rid of the loop altogether.
M = .1:.1:5;
u = M.* sqrt(y.*Ra.*Ta)
