Please ensure all the users whose membership has to be migrated to the target teams / groups 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 (whose membership has to be migrated to the target teams / groups) and retry the job.