Undefined function 'function_name' for input arguments of type 'double'

I'm solving a convex optimization problem(for matrix) in which I'm using 2 functions, one for objective function and another for nonlinear constraint. When I run it via optimization toolbox, it shows "Undefined function 'f_name' for input arguments of type 'double'".Also, I passed the initial value of x as zero matrix of required size(which is 498 here). Is the starting value passed correctly?

Here is the function codes

function f= coheren(x)

 load ('Dictionary.mat','D');
 Z = (D)*(D');
 [V, U,W]=svd(Z);
 a = mtimes(U,V');
 b = mtimes(V,U);
 f = (norm(U-(a*(x')*(x)*b)))^2;

end

function [c,ceq] = cohcon(x)

 load ('Dictionary.mat','D');
 [m,n]= size(D);%size(x)
 k = D*(D');
 T = (x)*k*(x');
 [V1,U1,W1] =svd(T);
 ceq = rank(U1)-m;
 c= [];

end

[Stephen23]

@Pushkar Khatri: please show us the complete error message. This means all of the red text.