You have a problem of dimensions at line 18:
At the first iteration, it holds: p = 1, q = 1, J = 2
pilotdata(1,2) is a scalar value (dimension 1 x 1) but k = 1 : 12
Mod(p,k) is a vector of dimension 1 x 12
You cannot assign this value to a scalar value.
What was your objective here ?