Bug in property or method error?

Try this expression for Stroke Width:

strokeWidth = effect("Stroke Width")("Slider");
taperOut = effect("Taper Out")("Slider");
groupIndex = thisProperty.propertyGroup(3).propertyIndex;
totalGroups = thisLayer("Contents")("Duplicate Groups")("Contents").numProperties;
strokeTaper = strokeWidth / totalGroups * (totalGroups / groupIndex);

easeIn(taperOut, 0, 100, strokeWidth, strokeTaper);

 I don't use var because it is redundant. 

The code should work as long as you have Effects Control Sliders named "Stroke Width" and "Taper Out" on the shape layer.

The tutorial is a little long-winded, and unfortunately, the author uses a paid 3rd party script to help write the expressions, so users that have not purchased the same tool don't see what is on the screen, 

The tutorial is not exactly how I approach the problem of creating a tapered stroke. It's a little clunky, but it should work. The workflow is viable if you cannot update to a version of AE that offers Tapered Strokes.

Again - technically the syntax is not correct. Anything beyond that would requzire to know what tutorial you are actually referring to.

Mylenium

The slider references don't make any sense. The way they are written "Effects" only makes sense if there is an effect of such name or a pseudo-effect. This is not AE's fault. You're missing something critical about the tutorial or it's simply not explained properly. Find a better tutorial.

Mylenium