mount.nfs : requested NFS version or transport protocol....

is not supported.

Oh no not another nfs question, but yes it is:

On a freshly formatted 120GB SSD I installed ubuntu 18.04 LTS udated to the latest version.
 Once runninng, I added Gufw and then installed the current version of NFS see here:

ea@discovery:~$ sudo apt install nfs-kernel-server
[sudo] password for ea:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  nfs-kernel-server
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 93.8 kB of archives.
After this operation, 345 kB of additional disk space will be used.
0% [Working]y
Get:1 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 nfs-kernel-server amd64 1:1.3.4-2.1ubuntu5.2 [93.8 kB]
Fetched 93.8 kB in 11s (8,782 B/s)
Selecting previously unselected package nfs-kernel-server.
(Reading database ... 178135 files and directories currently installed.)
Preparing to unpack .../nfs-kernel-server_1%3a1.3.4-2.1ubuntu5.2_amd64.deb ...
Unpacking nfs-kernel-server (1:1.3.4-2.1ubuntu5.2) ...
Setting up nfs-kernel-server (1:1.3.4-2.1ubuntu5.2) ...
Job for nfs-server.service canceled.
Job for nfs-server.service canceled.
invoke-rc.d: initscript nfs-kernel-server, action "restart" failed.
● nfs-server.service - NFS server and services
   Loaded: loaded (/lib/systemd/system/nfs-server.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Wed 2019-10-30 10:27:45 PDT; 4ms ago
  Process: 2329 ExecStopPost=/usr/sbin/exportfs -f (code=exited, status=0/SUCCESS)
  Process: 2326 ExecStopPost=/usr/sbin/exportfs -au (code=exited, status=0/SUCCESS)
  Process: 2325 ExecStartPre=/usr/sbin/exportfs -r (code=exited, status=1/FAILURE)

Oct 30 10:27:45 discovery systemd[1]: Starting NFS server and services...
Oct 30 10:27:45 discovery exportfs[2325]: exportfs: Failed to stat /mass2: No such file or directory
Oct 30 10:27:45 discovery exportfs[2325]: exportfs: Failed to stat /mass1: No such file or directory
Oct 30 10:27:45 discovery exportfs[2325]: exportfs: Failed to stat /mass0: No such file or directory
Oct 30 10:27:45 discovery exportfs[2325]: exportfs: Failed to stat /mass: No such file or directory
Oct 30 10:27:45 discovery systemd[1]: nfs-server.service: Control process exited, code=exited status=1
Oct 30 10:27:45 discovery systemd[1]: nfs-server.service: Failed with result 'exit-code'.
Oct 30 10:27:45 discovery systemd[1]: Stopped NFS server and services.
Failed to restart nfs-kernel-server, ignoring.
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for ureadahead (0.100.0-21) ...
ureadahead will be reprofiled on next reboot
Processing triggers for systemd (237-3ubuntu10.31) ...
ea@discovery:~$

Then I tried on another computer my now famous mount command:
sudo mount -t nfs -o proto=tcp,port=2049 192.168.0.104:/media/mass /media/mass
Password
then the above sentence
mount.nfs : requested NFS version or transport protocol is not supported

The firewall was set identical to the clients, a setting that always was correct