libsnmp-base breaks the ubuntu upgrade
Aptitude may help you to fix the issue. If it is installed please use it as follows
sudo aptitude dist-upgrade
to get newest dependencies on existing system and then follow its recommendation for possible solution.
If it is not installed please install it by sudo apt-get install aptitude and retry with above command.
And only when all dependency issues are fixed on 18.04 LTS you can run upgrade procedure to 20.04 LTS using sudo do-release-upgrade (in GUI) or update-manager -c (on desktop) .