FWIW I just tested the v18.2 WordPress appliance myself (LXC container on Proxmox v8.2.4). I used the password TurnKey12? and I could log into WP using the username admin and that password so I'm not sure what's wrong?!

Could you try running this (replace the email and domain with yours):

/usr/lib/inithooks/bin/wordpress.py --domain="www.example.net"  --email="admin@example.net" --pass="TurnKey12?"

Then try logging in again as admin with the password 'TurnKey12?' (no quotes).

Please let me know how you go.

If you continue to have issues, you can enable logging by editing the /var/www/wordpress/wp-config.php file and adding this code snippet:

define('WP_DEBUG', true);
if (WP_DEBUG) {
        @error_reporting(E_ALL);
        @ini_set('log_errors', true);
        @ini_set('log_errors_max_len', '0');
        define('WP_DEBUG_LOG', true);
        define('WP_DEBUG_DISPLAY', false);
        define('CONCATENATE_SCRIPTS', false);
        define('SAVEQUERIES', true);
}

Then restart apache:

systemctl restart apache2

If there are WordPress errors, then a new 'debug.log' file should be created in /var/www/wordpress/wp-content/ - e.g. to view in in terminal:

cat /var/www/wordpress/wp-content/debug.log

Note that it will only create that file if there are WordPress errors. So if you get a "No such file or directory" error, then WordPress isn't erroring.

Note: to disable the debug logging again, edit the first line of the code you copy/pasted to wp-config.php and change the value of WP_DEBUG to false. I.e. the fist line should now look like:

define('WP_DEBUG', false);

The rest of the code can stay as is (it will only be triggered when WP_DEBUG is true).