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@sampattuzzi
Hi ,
You Said We Don’t Need To Declare The Function in Header, But I Am Getting Error If I Don’t Declare it In Header,
GetLifetimeReplicatedProps(TArray< FLifetimeProperty >& OutLifetimeProps) const
See The Error :
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Also Let You Know The In Previous Lecture Same Case Was There For -
bool Server_MoveForward_Validate(float Value);
In Your Case How That Worked?? 
These are not build errors. What is the output from the actual build log?