Turn off "You have specified an invalid database connection group."

[eluser]summery[/eluser]
The best way might be to specify a valid database connection group.

Read about establishing database connections in CI.

Two basic things to check for in your config/database file:
- is $active_group variable set to a valid array of config variables? Default, perhaps?
- are your database config variables correct - hostname, db name, etc?

Good luck!