package mysql-server-8.0 8.0.21-0ubuntu0.20.04.3 failed to install/upgrade: el subproceso instalado paquete mysql-server-8.0 script post-installation devolvió el código de salida de error 1

Status changed to 'Confirmed' because the bug affects multiple users.

According to apport logs you had mysql-server installed at:

Start-Date: 2020-08-01 09:13:37
Commandline: apt install mysql-server-core-8.0
Requested-By: manuel (1000)
Install: ...

And 2 days later you installed mysql-client-core:

Start-Date: 2020-08-03 09:24:59
Commandline: apt install mysql-client-core-8.0
Requested-By: manuel (1000)
Install: mysql-client-core-8.0:amd64 (8.0.21-0ubuntu0.20.04.3)
End-Date: 2020-08-03 09:25:04

and then:

Start-Date: 2020-08-03 09:31:59
Commandline: apt-get install mysql-server
Install: ...

When install the mysql-server-8.0 package the log shows:

Configurando mysql-server-8.0 (8.0.21-0ubuntu0.20.04.3) ...
update-alternatives: utilizando /etc/mysql/mysql.cnf para proveer /etc/mysql/my.cnf (my.cnf) en modo automático
Renaming removed key_buffer and myisam-recover options (if present)
mysqld will log errors to /var/log/mysql/error.log
mysqld is running as pid 21738
dpkg: ... exit code 1

and looking specifically to mysql-server logs, it looks like it was correctly installed:

2020-08-03T14:35:42.017991Z 0 [System] [MY-013169] [Server] /usr/sbin/mysqld (mysqld 8.0.21-0ubuntu0.20.04.3) initializing of server in progress as process 20452
2020-08-03T14:35:43.242835Z 1 [System] [MY-013576] [InnoDB] InnoDB initialization has started.
2020-08-03T14:37:22.177611Z 1 [System] [MY-013577] [InnoDB] InnoDB initialization has ended.
2020-08-03T14:37:41.687358Z 6 [Warning] [MY-010453] [Server] root@localhost is created with an empty password ! Please consider switching off the --initialize-insecure option.

and 2 minutes later it was shutdown by you:

2020-08-03T14:39:58.601233Z 6 [System] [MY-013172] [Server] Received SHUTDOWN from user boot. Shutting down mysqld (Version: 8.0.21-0ubuntu0.20.04.3).

After that, it starts again:

2020-08-03T14:41:50.086925Z 0 [System] [MY-010931] [Server] /usr/sbin/mysqld: ready for connections. Version: '8.0.21-0ubuntu0.20.04.3' socket: '/tmp/tmp.kIFFUPPj6Z/mysqld.sock' port: 0 (Ubuntu).

and then

2020-08-03T14:41:51.318391Z 0 [System] [MY-013172] [Server] Received SHUTDOWN from user <via user signal>. Shutting down mysqld (Version: 8.0.21-0ubuntu0.20.04.3).

it is shutdown again... from that time and on mysql starts receiving a can't bind socket error message:

2020-08-03T14:42:19.395575Z 0 [ERROR] [MY-010262] [Server] Can't start server: Bind on TCP/IP port: Address already in use

likely because the socket opened by the previous instance is still in FIN_WAIT mode, so you have to wait for it to complete before being able to respin the instance again to the same port.

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

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