The problems are too many to specifically enumerate:
I have no idea what the ‘(iii)’ subscript refers to (I am guessing ‘l_vector’) however it cannot just sit there at the end of the statement, since it does not specifically refer to anything.
I have no idea if this will work and do the calculation you want, however the parentheses and function calls all appear to agree:
Ms(iii) = 2/pi*(w1*log(tan(acos(w1/l_vector(iii)))+l_vector(iii)/w1)+l_vector(iii)*(pi/2)-l_vector(iii)*acos(w1/l_vector(iii)));
Note the changes. Make appropriate edits to get the result you want.