'introScreen was not declared in the scope

If you ask specific questions, we can provide assistance. If you want someone to fix it for you, then go to the Gigs and Collaborations category.

Did you write this code yourself? The basic structure of the program has problems and you ask about an advanced topic- interrupts?
Fix your structure problems before moving on.
(An interrupt, by the way, is simply a function that gets called by an external event.)