The error message is indicating that the folder the plugin would install to already exists, so as far as WordPress is concerned, the plugin is already installed.
If it is not showing in your plugins panel, then either the main plugin file was deleted but not the folder, or something else has gone wrong when it was removed.
In order to correct the issue, the easiest way is going to be use an FTP program to access your site’s file system, then browse to the /wp-content/plugins/ directory and delete (remove) the plugin’s folder (in this case /all-in-one-seo-pack/).
Once that is removed, you can (re)install the plugin.
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