What is wrong in this code? trying to code the equation and j is time instant

N= 10;
M = 4;
data = randi([0,3],999,1);
u = pammod(data,M);
x= zeros(24,24);%estimated 
for i = 1:N
    syms  i
    for j = 1:N
        
        H(:,k)= symsum ((x(i(:,j)))*(x(i+N(:,j))),i,1,N); 
    end
end