The error is clear, you are trying to save more number of elements in the initilaized matrix.
Example:
A = zeros(3,3) ; % initialize A to 3x3 zeros matrix, to save data later
A(:,1) = rand(1,3) ; % no error, because column should have three elements and we have three here
A(:,2) = rand(1,5) ; % error, because column should have three elements, but you are saving five elements, This is not allowed
So, similiarly in your case, U_RK is a array with 8 rows, where you should save 8*1 array; but the RHS is giving you an array of size 8*1001.
Check the dimensions of your code and then initialize the matrix properly.
