Sounds like it isn't being loaded. What happens when you log this out?
console.log(com.greensock.plugins.ScrollToPlugin.version); // -> 1.8.0
If that logs a version number out, it's loading correctly. From there I would try it without wrapping the window in jQuery.
solutionsTl.to(window, .8, { scrollTo: {y: scroll}, ease: Circ.easeOut}, "-=.5");
If that works, check how you're loading jQuery. You might also want to try putting your scripts at the bottom of your page, right before the closing </body> tag instead of in the head. The only time I put scripts in the head is if I'm using a framework like Angular, where it has to run before anything can be displayed.