is there an AE plugin for triggering layer visibility with audio and expressions?

That is a very well-explained tutorial. The only potential problem that I see is that there may be some punctuation missing in the if/else statement. A more standard way of controlling a variable looks like for opacity looks like this:

beatCounter = thisComp.layer("Controls").effect("Beat Counter")("Slider");
if (beatCounter == index - 1){
  t = 100;
else {
  t = 0;
}
t

The way the author wrote the expression failed to add a semicolon to the end of the if and else arguments. That could cause the expression to fail. Adding the opening and closing brackets also helps clear things up.

If that does not help we need to see your timeline with the modified properties of at least one problem layer showing. Select the layer, press 'uu' (the "u" key twice), then take a screenshot and drag it to the reply field or embed it using the Tool Bar and make it big. We also need to see the Audio Amplitude Graph editor with the proper channel, probably "Both Channels" selected so we can check the values.

Trapcode Sound Keys, which is part of the Trapcode Suite, is an improved Convert Audio to Keyframes tool that takes a little of the work out of controlling layer properties, but you still have to have a basic understanding of expressions.

Try downloading the project file in the YouTube description. If it also throws the error the if/else statement punctuation is the problem.