Why Compare or Repair is not supported from a Materialized View on source to a Materialized View on target?
A Materialized View is a database object that is derived from another database object also termed as base object. So while it is quite ok to have a table on target that can be synchronized with the content of a Materialized View from the source, the same cannot be said for a Materialized View on the target. Even if a DML is done on the Materialized View on the target, it will be lost when the view is refreshed the next time. So logically it is not a good idea to alter the content of a Materialized View on the target via some DML, as Repair would do it. For the same reasons, the Compare does not support this either.