I can expand all mailboxes but i can't see the users mailboxes is it only viewable in restore more?
Yes. You can only see the mailboxes when you are doing a restore.
Backup successfully completed but when I tried to restore one single mailbox I expand the mailbox and then the backup database but i couldn't see the users mailboxes!! and it gave me 0 KB as shown in below attachment.
Try backing up the Exchange server on the active node, rather than the DAG.
Can you please explain to me how?
Here are the best practices:
The Admin Guide will also have further information.
If you find this is a solution, please mark it as such.
If the participating servers in a DAG do not show, see this document.