I assume that the message that you are missing should read
This was likely caused by:
Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.
This is the case on your system. You have added software from outside
Ubuntu, namely the oibaf PPA.
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https://bugs.launchpad.net/bugs/1923080
Title:
ubuntu upgrade to 20 from 18 .04.5 fails. Error message is
indeterminate.
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