seems that you haven't declared an object named "serial"!
Using external librarries you first have to add the library(s) to the project using <tools> <components...>.
After that, you have to add the objects you are accessing within your project using <tools> <add object>. Select the right type of object ("Serial" in this example) and name it like you access it in your program ("serial" in this example).
Another way is to add the object during runtine with AddObject(Object Name, Object Type).