This line:
sin(theta_b) = 1/sqrt(1+er1/er2);
is not valid syntax because you cannot assign a value to sin(theta_b). If you want to assign a value to theta_b such that the mathematical relation above is true, you could do this:
theta_b = asind(1/sqrt(1+er1/er2));
which is basically "solving for" theta_b in terms of the other variables, using the inverse sine function asind(), which returns an angle in degrees.
And since you're dealing with angles in degrees here, you should use sind and cosd, instead of sin and cos, which expect input angles to be in radians.
I made those changes and a few others below:
er1 = 2.55;
er2 = 1;
eta_1 = 377;
eta_2 = 377/sqrt(er1);
% sin(theta_b) = 1/sqrt(1+er1/er2);
% cos(theta_t) = sqrt(1-er1*sin(theta_b)^2/er2);
theta_b = asind(1/sqrt(1+er1/er2));
theta_t = acosd(sqrt(1-er1*sind(theta_b)^2/er2));
% for theta_i = [1:90];
% parallel_pol = (eta_2*cos(theta_t)-eta_1*cos(theta_i))/(eta_2*cos(theta_t)+eta_1*cos(theta_i));
% end
theta_i = 1:90;
parallel_pol = (eta_2*cosd(theta_t)-eta_1*cosd(theta_i))./(eta_2*cosd(theta_t)+eta_1*cosd(theta_i));
% plot(theta_i,abs(parallel_pol),"-","r","LineWidth",2)
plot(theta_i,abs(parallel_pol),"-r","LineWidth",2);
xlabel('theta_i');
ylabel('parallel\_pol');
