Hi Victor,
it means you don’t have a configuration.php file which Joomla! assumes it’s not installed and looks for the “installation/” folder, which of course does not exist either. Please follow these steps below :
1. Rename configuration.php-dist to configuration.php.
2. In the file find the db, user, password fields a few lines down and re-enter all the correct database info that applies to your website. Save the corrections, and overwrite the existing file ( make sure it is configuration.php ).
3. Delete the installation folder or rename it something else and recheck.