Couldn't install Ubuntu 18.04 on WSL

Did you try uninstall the Windows 10 Subsystem for Linux then try re-enabling it again then go through the download procedures?

Just tried it. Still the same. 

Uninstalled Ubuntu 16,

Turned off the WSL feature,

Restarted comptuer

Turned on the WSL feature

Tried installing Ubuntu 18. 

Could you try doing it through the command line instead as described in the following article?

https://www.howtogeek.com/278152/how-to-update-...

Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.

It installed only 16.04. I tried this command sudo do-release-upgrade from bash. It didn't find any updates too. 

I am trying to install this btw Ubuntu 18.04

sudo do-release-upgrade -d 

will work.

As per @X.ArthurM.   "sudo do-release-upgrade -d" works.

... Just know that the release upgrade process takes a VERY VERY VERY VERY LONG TIME. 

Make sure you're on AC power (not battery), and it's best to be on a fast internet connection.

During the upgrade, whenever it's detected that you made system configuration changes to the default configurations of a package, the installer will ask whether you want to keep your changes, or overwrite your changes with the default config of the upgrade, or sometimes it will let you do a merged install.   If you made many system changes, expect to be asked about most/all of them.