Able to ping two linux machines but cannot mount nfs share from one to another
Specify the host name of the ubuntu server before the : in your command.
For RedHat, but the idea is the same: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/System_Administration_Guide/s1-nfs-mount.html
If the host name is myubuntu and share is /export/nfs then it should be mount myubuntu:/export/nfs not just mount :/export/nfs
Also, make sure you have iptables open and not blocking the ports.