In Kali Linux, you will need to start up the postgresql server before using the database:

$ systemctl start postgresql

After starting postgresql, you will need to create and initialize the msf database with msfdb init:

$ msfdb init

If you just created a new exploit and want to refresh metasploit db to start using the newly created exploit, enter the following:

$service postgresql restart && msfdb reinit

Then, type the following command:

$msfconsole -q

The -q will start msfconsole in debug mode, so if you made mistakes in your new exploit class, the debugger will then print it to the screen.