You are supposed to change the /etc/apt/sources.list file and replace the name of your previous release. On 18.04.4 replace every instance of bionic with focal. Just run simple ‘sed’ command performing replacement$ sudo sed -i ‘s/bionic/focal/g’ /etc/apt/sources.list

Source: LXer – How to Upgrade Ubuntu 18.04 to 20.04 LTS Focal Fossa via Debian way (right now)