mount: mount to NFS server failed: RPC Error: Program not registered.

Issue:
[root@linux01 ~]# mount -t nfs linux02:/u01/app /u01/app
mount: mount to NFS server 'linux02' failed: RPC Error: Program not registered.

[root@linux02 ~]# showmount
mount clntudp_create: RPC: Program not registered

Solution:
the nfs service was down on the source server so started the same and able to mount the drive successfully in the destination server.

[root@linux02 ~]# service nfs start
Starting NFS services:                           [  OK  ]
Starting NFS daemon:                             [  OK  ]
Starting NFS mountd:                             [  OK  ]

[root@linux01 ~]# mount -t nfs linux02:/u01/app /u01/app