The version of MATLAB Coder that you have supports generating code for matrices of unbounded size (it generates C code that calls malloc/free to allocate memory for those matrices). Your version of Simulink does not have that feature -- it wants all matrices to have a known upper bound.
You can either upgrade to a newer release (as of now, currently shipping release of Simulink does not have that limitation), or add hint in the code, helping Simulink to know what is the biggest size of that matrix.
the bottom line: add
assert(len < 1000);
before the zeros(len, 1)
and that should help.