I am on Ubuntu 18.04, after running the initial install command, I am always getting this ‘Error: Docker not installed or not running.’
I can’t create site. Is it because of Ubuntu Version. If 18.04 is not supported, why? 16.04 is already over 2.5 years old.
Are you running it on an OpenVZ enviroment? If so I had the same issue using it in an OpenVZ environment where the kernel is shared. Switched with my hosting company to KVM and all is well.
Running the command below will show you that docker is in fact not running.
sudo systemctl status docker
or
sudo service docker start
I get ‘Unit docker.service could not be found.’
And its Linode, so KVM, I double checked after your post. But I am in Ubuntu 18, maybe that’s what causing the issue.
Did you try installing Docker? I’m on 18.04 and don’t have this issue. I even upgraded from 16.04 to 18.04.
I just installed it. Still nothing happening here. i don’t want to manually configure Docker.
ee cli self-uninstall --yes then install EE again.
Can you add your install log?
FWIW, I stopped using Linode a long time ago because their OS images are utterly terrible compared to mainline distributions found on DigitalOcean, Azure, AWS etc. A lot of components are oddly missing. The only good thing about Linode is Lassie 
Might be worth trying your same steps on another provider such as DigitalOcean or Lightsail. If you get through, it’s definitely Linode’s issue and you would have narrowed it down without much effort.
If your steps don’t work on another provider, then you can look into the EE configuration logs more deeply, since EE4 otherwise fully supports Ubuntu 18.04 LTS.