Can't migrate standalone MongoDB to Cluster
Logged into mongodb, to the admin database, with a user who has the following roles: clusterMonitor, readAnyDatabase, userAdminAnyDatabase


...


  not authorized on admin to execute command { replSetInitiate: undefined, $db: \“admin\” }“


This error is expected since clusterMonitor is a read-only role.

To initialise or update replica set configurations when access control is enabled your user needs a cluster management role like clusterManager or clusterAdmin.