In the function Array, the size of the index J does not match the size of the data to be re-indexed. If you ignore, the data will be truncated or padded with null values to match the length of the index.
Suppose you have an index defined:
Index J := [1, 2, 3, 4]
Then if you evaluate the expression:
Array(J, ['a', 'b'])
the warning will result, since J has 4 elements and you've supplied only two items. In this example, the result will be an array indexed by J, but with the last two elements set to Null.
If this is as you intended, then ignore, or better, place the expression inside