Hello Dimitrios,
One way to force MATLAB to solve the given equation so that you can get the desired result is that try to rearrange the equation and then provide it to MATLAB.
You can write 
as 
as So your code will be -
syms theta mu_s positive
assume(theta > 0 & theta < pi/2)
% rearrange the equation
eq = tan(theta) == mu_s;
sol = solve(eq, theta, 'Real', true, 'ReturnConditions', true, 'PrincipalValue', true);
simplify(sol.theta)
By doing this, you will get solution as - 
Hope this helps!
