How do I resolve the error "The signal at output port 2 of 'ftranssine2freq/MATLAB Function/ SFunction ' is a variable-size signal with a nondiscrete sample time.."

Hello everyone I was trying some simple code to find fourier transform of two cosines of different frequency numerically i.e., without using the function fft() as my guide has advised me to calculate it numerically for some specific purpose. after running the codes I am getting following error: The signal at output port 2 of 'ftranssine2freq/MATLAB Function/ SFunction ' is a variable-size signal with a nondiscrete sample time.

function y = fcn(z)

%#codegen

y=z;

N=250;

ts=.002;

u=[0:N-1]*ts;

k=0;

for f=0:1:800

k=k+1;

y(k)=real(trapz(u,z.*exp(-1i*2*pi*f*u)));

end

plot(f, abs(y))