Please ensure all the users whom you want to add in the migrated target chats are available in the “Application Users Mapping” which is placed in the Settings -> Users Mapping tab. Post adding the missing mappings, retry the job.

(OR)

Post the migration is complete, please open Migrate -> Migration Jobs -> Edit Job Settings for the problematic job. Under section Mapping Not Found Users, map the target users (whom you want to add in the migrated target chats) and retry the job. This would resolve the “Users Mapping Not Found” issue.