How to enable Debug and Ratchet?

Sorry for the noob question.

I see many times in issues the line for enable debug. In line 30 in WebSockets.h
But, i dont understand what i need put in line 30 for debug works.
I tried Serial, Serial1, Serial2, Serial3... But no ones works. In all cases i got an error in compilation.

I need to debug because Arduino is not connecting the WebSocket server made in Ratchet (PHP).
I see the connection in Ratchet console, but Arduino disconnects immediately following.