When a WordPress site is correctly installed within the DB, but the wp-config.php that WP-CLI parses provides the wrong DB prefix, WP-CLI aborts any actions with the following message:

Error: The site you have requested is not installed.
Run `wp core install`.

In the particular case I encountered, this was due to the wp-config.php incorrectly parsing the prefix from a .env file. See related issue.

Although this should normally not happen, it would be preferable for WP-CLI to check the existing tables when the DB connection was successful, any maybe point the user to the fact that there is already an install under a different prefix, hinting at a configuration issue.