Then it must be with Credentials you entered in wp-config.php
Please check wp-config.php for DB name, Username & Password.
verify that user is permitted to operate on Database
Can you check if Mysql is running on your server ?
Usually its
1. MySQL Server is not working
2. Invalid Credentials
3. Username, Password and Database names are correct. But User is Not Permitted to perform operations on Database.
As you are saying you are able to install WordPress from Softaculous Successfully. Then you should not get this error ! Can you Check with your Web Host?
