1. The database and user did not create successfully.
2. Database and user are not connected with each other.
3. If you use xammp localhost, then it could be a xammpp version problem.
4. Hostname by default should be localhost but some server uses their own hostname. So please your hostname from the server provider.
5. It could be your server permission issue.
6. If you are using Linux server please check this tutorial:https://www.youtube.com/watch?v=0oYCK99xnvY
404 error or htaccess or 403
1. If you are having 403 error then check this blog here: https://blog.bdtask.com/how-can-i-solve-404-not-found-after-installation/