Hi @Keke Yuan ，
When starting the synchronization for database mirroring, you may get the above error, mirroring requires a full restore and at least one tlog restore, the error is dur to the mirror database has insufficient transaction log data
To start the synchronization successfully, you can do following steps:
Principal Instance - - Take a full backup and a log backup as well, Copy the full/log backups from Principal Instance to Mirror instance
Mirror Instance - - Restore with NORECOVERY option the full backup
Mirror Instance - - Apply the log backup
Principal Instance - - Start synchronization
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