he packages of Ubuntu is synced on foreman GUI but cannot register the Ubuntu client on foreman GUI. can anyone please help me on how to do this any link or suggestions ?
You can find the subscription-manager at https://apt.atix.de/ (or at least for the latest Ubuntu and Debian release)
I use versions 16.04 and 18.04. And how do you install the agent?
For older versions you would have to build the packages on your own at the moment.
The required files are in https://github.com/candlepin/subscription-manager/.
There are reports for the 20.04 package also be working on 18.04 but not 16.04.
Do you train any tutorial?
Unfortunately no, I am more a RPM guy myself and not very firm with DEB.
I would hope, that checkout of the git and something like
dpkg-buildpackage -uc -us would be enough.