MATLAB: What does this mean

invalid operator

fprintf('\nIteration number = %d \n', m);

↑

Error: Invalid use of operator.

How can I fix this?

fprintf('\nIteration number = %d \n', m);

Personally, I'd use:

    fprintf(" Old Means: (%s)", strjoin(compose('%.4f', M(:, :, i)), ','));

: is the colon operator. As per the documentation:

 A(:) is all the elements of A, regarded as a single column.

So, your line just reshapes d as a single column. It is equivalent to

d = reshape(d, [], 1);