Comment 2 for bug 1958641

Thank you for taking the time to file a bug report.

Looking at your DpkgTerminalLog.txt file, I see the following excerpt:

Setting up mysql-server-8.0 (8.0.27-0ubuntu0.20.04.1) ...
update-alternatives: warning: alternative /etc/mysql/mariadb.cnf (part of link group my.cnf) doesn't exist; removing from list of alternatives
update-alternatives: warning: alternative /etc/mysql/my.cnf.fallback (part of link group my.cnf) doesn't exist; removing from list of alternatives
update-alternatives: warning: /etc/alternatives/my.cnf is dangling; it will be updated with best choice
update-alternatives: using /etc/mysql/mysql.cnf to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Renaming removed key_buffer and myisam-recover options (if present)
ERROR: Unable to start MySQL server:
mysqld: Can't read dir of '/etc/mysql/conf.d/' (OS errno 2 - No such file or directory)
mysqld: [ERROR] Stopped processing the 'includedir' directive in file /etc/mysql/my.cnf at line 20.
mysqld: [ERROR] Fatal error in defaults handling. Program aborted!
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-8.0 (--configure):
 installed mysql-server-8.0 package post-installation script subprocess returned error exit status 1

This tells me a few things:

1) You probably had (or still have) MariaDB installed. MySQL will conflict with MariaDB and they will not play well together. I would recommend that you purge your MariaDB installation before proceeding with the MySQL one. However, if you already have both MySQL and MariaDB installed and you purged MariaDB, this will have the side effect of purging files that are actually needed by MySQL (for example, configuration files; see below). In this scenario, when you upgrade MySQL after having purged MariaDB, you will unfortunately see errors like you saw above.

2) mysqld (MySQL's daemon) expects to find the directory /etc/mysql/conf.d/, but in your scenario the directory did not exist. This was probably caused by purging MariaDB before upgrading the MySQL package.

Since it seems likely to me that this is a local configuration problem,
rather than a bug in Ubuntu, I am marking this bug as 'Incomplete'.

However, if you believe that this is really a bug in Ubuntu, then we would
be grateful if you would provide a more complete description of the problem
with steps to reproduce, explain why you believe this is a bug in Ubuntu
rather than a problem specific to your system, and then change the bug
status back to "New".

For local configuration issues, you can find assistance here:
http://www.ubuntu.com/support/community