Ubuntu 18.04 to 20.04 Error: Please install all available updates for your release before upgrading

  $ sudo do-release-upgrade

Checking for a new Ubuntu release

Please install all available updates for your release before upgrading.

$ sudo apt-get upgrade

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Calculating upgrade... Done

The following packages have been kept back:

  linux-generic linux-headers-generic linux-image-generic

0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.

$ sudo apt-get dist-upgrade

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Calculating upgrade... Done

The following packages have been kept back:

  linux-generic linux-headers-generic linux-image-generic

0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.

$ apt list --upgradable

Listing... Done

linux-generic/bionic-updates,bionic-security 4.15.0.135.122 amd64 [upgradable from: 4.15.0.34.36]

linux-headers-generic/bionic-updates,bionic-security 4.15.0.135.122 amd64 [upgradable from: 4.15.0.34.36]

linux-image-generic/bionic-updates,bionic-security 4.15.0.135.122 amd64 [upgradable from: 4.15.0.34.36]

$ dpkg --get-selections | grep hold

linux-headers-generic hold

linux-image-generic hold

$ sudo apt-get install linux-headers-generic

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following additional packages will be installed:

  linux-headers-4.15.0-135 linux-headers-4.15.0-135-generic

The following packages will be REMOVED:

  linux-generic

....

$ sudo apt-get install linux-image-generic

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following additional packages will be installed:

  linux-image-4.15.0-135-generic linux-modules-4.15.0-

$ sudo reboot

 $ sudo do-release-upgrade  --> This updated my Ubuntu 18.04 to 20.04