How to make PM2 as a autostart service?
When configuring PM2 is finished and it's working properly, you must run two commands to make your PM2 as a autostart service and enjoy that.
1) Run 'pm2 save': It's save all of your PM2 configure to a file.

admin@server:~$ sudo pm2 save


2) Run 'pm2 startup': This command creates a system service and runs your last change saved with the first step. 

admin@server:~$ sudo pm2 startup


So, Run the first command when you changed the configuration to store the last changes.