Connect the CMS database
The public website can still show its original content, but the CMS needs the database file before edits can be saved.
What to check
Create db_strings.php in the website root. It should define $db_username, $db_pass, and $db_name. The CMS automatically uses localhost unless you also define $db_host.
db_strings.php was not found. Put it in the website root beside index.php, or one folder above the website root.