I suspect you already know this but I spent enough time on it that I don't want others to go through it. Installation of MySQL 8 on Ubuntu 20 fails if you have deprecated variables from an old version of MySQL. I had
query_cache_limit = 1M
query_cache_size = 16M
both of which are deprecated. The error does show up in the mysql error log but shows up as a failed start in mysql service which is tried over and over after re-boot.

https://dev.mysql.com/doc/refman/8.0...ars-deprecated

Just did not want others to lose hours like I did.
Johnny Wunder