The final substitution into U_2 needs to be changed as shown below.
syms('R', 'positive')
syms('C', 'positive')
syms('omega', 'positive')
Z_s = R + 1 / 1j / omega / C; % chnage j to 1j throughout
syms('U_2')
syms('U_C')
U_2 = 1 / 1j / omega / C / Z_s * U_C;
U_2 = simplifyFraction(U_2);
Z_p = 1 / (1j * omega * C + 1 / Z_s);
syms('U_1')
U_C = Z_p / (R + Z_p) * U_1;
U_C = simplifyFraction(U_C);
U_2 = subs(U_2); % this line changed, update U_2 with current value of U_C
U_2 = simplifyFraction(U_2)
