This is likely because you have python2 packages installed from a PPA.
You'll want to remove those python2 package and try the upgrade again.
These are some of the packages from the PPA.
python2 2.7.15-3~oibaf~b 1
python2-minimal 2.7.15-3~oibaf~b 1
The following page contains instructions about
** This bug is no longer a duplicate of bug 1874512
Can't upgrade 18.04 to 20.04
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https://bugs.launchpad.net/bugs/1874549
Title:
20.04 is not getting updated from 18.04
To manage
*** This bug is a duplicate of bug 1874512 ***
https://bugs.launchpad.net/bugs/1874512
** This bug has been marked a duplicate of bug 1874512
Can't upgrade 18.04 to 20.04
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