As the error says, there is a possible execution path on which mu1 is not defined. In particular, if all of the conditions in your if and elseif statements are false, then mu1,mu2,mu3 will not be defined.
To resolve this define them to default values either before the if:
mu1 = 0; mu2 = 0; mu3 = 0;
if s<= -1 ...
or add an else branch:
elseif s > a
mu1 = 0;
mu2 = 0;
mu3 = 1;
else
mu1 = 0;
mu2 = 0;
mu3 = 0;
end
