After you have restored the mirror database and configured Data mirroring, when you start the mirroring it throws this error.

The solution to resolving this is to restore the backup of the transaction log to the Mirror database.

1. Take backup of the TLOG at Principal site

BACKUP LOG TO DISK = N’E:\transaction_log_bkp\Tlog_Bkp.trn’ WITH NOFORMAT, NOINIT, NAME = N’Test-Transaction Log Backup’, SKIP, NOREWIND, NOUNLOAD, STATS = 10
GO

copy this backup to the Mirror site

2. Restore the TLOG backup to the mirror site.

RESTORE LOG FROM DISK = N’D:\transaction_log_backup\Tlog_Bkp.trn’ WITH FILE = 1, NORECOVERY, NOUNLOAD, STATS = 10
GO

Now restart the mirroring session and it should be ok.