Ubuntu server upgrade 18.04 LTS to 20.04 LTS


I have a production Ubuntu Server 18.04 LTS on DigitalOcean. I tried to upgrade it to 20.04 LTS, but after “sudo do-release-upgrade -d” I got this:

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Hit http://repo.mysql.com/apt/ubuntu bionic InRelease
Hit http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit http://ppa.launchpad.net/certbot/certbot/ubuntu bionic InRelease
Hit https://repos.insights.digitalocean.com/apt/do-agent main InRelease
Hit http://ppa.launchpad.net/nginx/stable/ubuntu bionic InRelease
Hit http://mirrors.digitalocean.com/ubuntu bionic InRelease
Hit http://ppa.launchpad.net/ondrej/nginx/ubuntu bionic InRelease
Hit http://mirrors.digitalocean.com/ubuntu bionic-updates InRelease
Hit http://ppa.launchpad.net/ondrej/php/ubuntu bionic InRelease
Get:1 https://deb.goaccess.io bionic InRelease (1,629 B)
Fetched 1,629 B in 0s (0 B/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done

Sorry, no more upgrades for this system

There will not be any further Ubuntu releases for this system's
'i386' architecture.

Updates for Ubuntu 18.04 will continue until April 2023.


Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
=== Command detached from window (Mon Oct 12 06:01:54 2020) ===
=== Command terminated with exit status 1 (Mon Oct 12 06:02:04 2020) ===

What can be the reason for the upper problem?