This issue appears quite a few times on this forum. I got stuck with this for quite a few hours recently ( have not used concrete5 for about 2 years now). So for the next noob that comes along;

Pls ensure ( besides getting username, password correct) that;

- Mysql / MariaDB is already running
- Server - localhost or 127.0.0.1 or IP ( socket 3306 or otherwise )

And ....

- an empty database ( create it first before running concrete5 via phpmyadmn etc ).

I got stuck on that last bit. Cant seem to locate any line in the install documentation that
says an empty database must be available first. Hehehehehe....