Read the error. Think about what it says. Think about what it means. Look into the reasons it might be saying that.
Check all of the pieces of your database connection - host name or address, user name, password, privileges, database server status.
This is the down side to using "all-in-one" control panel installations. My experience has been that they generally don't work right out of the box. Maybe they do for some people, but not for me. I've had that experience with ISPConfig and Virtualmin. After giving up on ISPConfig, I installed Virtualmin and still spent a week troubleshooting the postfix configuration that Virtualmin installed. In the future, I'll probably skip the control panel and do it all myself since I now have a much better working knowledge of all the nuts and bolts than I did before.