There's another sentence in that validation message.
"You must either match the grain of all the hierarchies under the dimension to the measure dimension grain, confirm that the level key is a surrogate key, or remove the aggregation rule."
I guessin...
"You must either match the grain of all the hierarchies under the dimension to the measure dimension grain, confirm that the level key is a surrogate key, or remove the aggregation rule."
I guessin...