So if you have had issues with U2F usage specifically the Yubico U2K Key on Ubuntu then a quick save is here !
I would highly recommend using Chrome and not Firefox for a start, as there is a handy extension you will need from the Chrome store here.
Restart your Chrome browser to enable the plug in.
Then you will need to create a file called 70-u2f.rules
sudo nano /etc/udev/rules.d/70-u2f.rules
and then add this EXACTLY as it is into the editor - get the text from GitHub
Save the file and now restart your Ubuntu server or desktop.
Then proceed with your Google 2-step authentication.
* again as all things on the internet, use are your own risk !