To resolve it I changed the fdbmonitor.conf file to have details of backup_agent, because it seems the backup_agent is not being started by default. So insert below snipped in your fdbmonitor.conf file.
[backup_agent]
command = /usr/bin/backup_agent -C <fdb-cluster-file >
[backup_agent.1]
in my case fdb-cluster-file is at /var/dynamic-conf/fdb.cluster. After that you dont have to restart the service because this dir is always listened and if anything is changed the processes will be loaded accordingly. After doing the mentioned changes I was able to take the backup successfully.