The Merge Agent failed because the schema of the article at the Publisher does not match the schema of the article at the Subscriber. This can occur when there are pending DDL changes waiting to be applied at the Subscriber. Restart the Merge Agent to apply the DDL changes and synchronize the subscription.
When the merge process is propagating changes from the Subscriber to the Publisher, it compares the version of the replication stored procedures that is being used by the Merge Agent with the version of the stored procedures at the Publisher. If a DDL change occurred while the merge process was running, a schema mismatch could be detected.
Retrying the merge process should fix the problem, because the Merge Agent will start using the version of the replication stored procedures present at the Publisher.