'AttachPinChangeInterrupt' was not declared in this scope
You don't have the library installed. To install:


Go to https://github.com/GreyGnome/PinChangeInt  
Click the green Clone or Download button  
Select Download ZIP


The .ZIP file will download to your computer. Take note of the location.


Open up the Arduino IDE
Go to Sketch > Include Library > Add .ZIP library...
Browse to the download location
Select the PinChangeInt ZIP file


Its done! Compile your program and all should work!