When you run a query against a table that has multiple repeated fields, you get the error something like below.
Error : Cannot output multiple independently repeated fields at the same time. Found column_a and column_b
If this is the case, you might want to
Flatten one of the fields.
FROM (FLATTEN([dataset.tableId], children))
(citiesLived.yearsLived > 1995) AND
(children.age > 3)
GROUP BY fullName, age, gender, citiesLived.place
Quoted from: https://cloud.google.com/bigquery/docs/legacy-nested-repeated#nested
Select fewer fields
You might want to select fewer fields. If you only care about getting flattened output of a few fields, you can remove the independently repeated fields from your query results by explicitly selecting only the fields you care about.
Quoted from: https://stackoverflow.com/questions/25583878/flattening-google-analytics-data-with-repeated-fields-not-working-anymore