How to work on Arduino Mega.

Hi I was trying to work the code with arduino mega.  The only change I did was use Hardware serial3 instead of software serial on pins 2 and 3. However when I run the code I keep  getting the following error: 
ARDUINO: Invalid CRC in a loop. From what I could understand the program is getting stuck at esp.ready function(). What does this mean and how can I correct this error.
