@Sally,

Try to a re-configure system with the following command will fix the error.

$ sudo dpkg --configure -a

If the above command is not fixed, try to force the package installation using the following command.

$ sudo apt install -f
OR
$ sudo apt install --fix-broken

dpkg: warning: old mysql-server-8.0 package pre-removal script subprocess returned error exit status 1
dpkg: trying script from the new package instead ...
Failed to stop mysql.service: Unit mysql.service not loaded.
invoke-rc.d: initscript mysql, action "stop" failed.
dpkg: error processing archive /var/cache/apt/archives/mysql-server-8.0_8.0.27-0ubuntu0.20.04.1_amd64.deb (--unpack):
new mysql-server-8.0 package pre-removal script subprocess returned error exit status 1
Failed to stop mysql.service: Unit mysql.service not loaded.
invoke-rc.d: initscript mysql, action "stop" failed.
Failed to start mysql.service: Unit mysql.service not found.
invoke-rc.d: initscript mysql, action "start" failed.
Unit mysql.service could not be found.
dpkg: error while cleaning up:
installed mysql-server-8.0 package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
/var/cache/apt/archives/mysql-server-8.0_8.0.27-0ubuntu0.20.04.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

will not install any other updates on the system whilst this loop remains.