Unable to add database in DAG

I have just created a Mailbox serve add made it DAG members when I went to add a database in DAG on this server got below error...
error
MailboxDB_Users2
The seeding operation failed. Error: An error occurred while running prerequisite checks. Error: The specified database isn't configured for replication and therefore cannot be used to perform seed operations. [Database: MailboxDB_Users2, Server: EXCH2013.CC.MUMBAI.COM]

I tried re-adding database but it is showing already member of that server and "Database" and "Context state" State is UNKNOWN.

Again I tried adding Database using command, giving below error......................

The seeding operation failed. Error: An error occurred while running prerequisite checks. Error: The specified
database isn't configured for replication and therefore cannot be used to perform seed operations. [Database:
MailboxDB_Users3, Server: EXCH2013.CCA.MUMBAI.COM]
    + CategoryInfo          : InvalidOperation: (MailboxDB_Users3:String) [Update-MailboxDatabaseCopy], InvalidDbForSe
   edSpecifiedException
    + FullyQualifiedErrorId : [Server=EXCH2013,RequestId=201e346a-c580-4964-aa52-b343506d0094,TimeStamp=7/26/2017
    4:04:13 PM] [FailureCategory=Cmdlet-InvalidDbForSeedSpecifiedException] EB97D1E5,Microsoft.Exchange.Management.Sy
  stemConfigurationTasks.UpdateDatabaseCopy
    + PSComputerName        : exch2013.tta.mumbai.com
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