If your master key is not protected by the service master key, you need to open it. Since I don't know how you configured your encryption piece, you may need an

open master key decryption by password = 'XXXX'

then you can issue a backup, but try this first:

BACKUP CERTIFICATE SQLServerTDECertificate

TO FILE = 'MySQLCert.cer'

WITH PRIVATE KEY (file='MySQLCertKey.pvk',

ENCRYPTION BY PASSWORD='!@chennuio89*()as@!')

Please also tell me that isn't your real password.