Hello,
You are likely seeing this error message in the 2nd step because there is no index on the date field based on which the Online Archive is getting configured.
From our documentation:
Atlas runs an index sufficiency query to determine the efficiency of the archival process. If the number of documents scanned to the number of documents returned is 10 or more, the query result triggers an
Index Sufficiency Warning. This warning indicates that you have insufficient indexes for an efficient archival process. For date-based archives, you must index the date field.
Once the index is created on the date field, you can rerun Step-2 and the error message should not be seen.
Thanks !