PulseSensorAmped Processing error

Hello~

i've Arduino UNO, Pulse sensor/
so just following guide book. ===>"PulseSensorAmpedGettingStartedGuide".
but i've problem. ==> " variable or field 'serialEvent' declared void"

how can i fix it?

Use serialEvent as a function that doesn't return a value.
Post your code.
Use code tags.
Post in a more appropriate section.