Easy Text Box - sourceRectAtTime Expression not working, pics included

That tutorial is flawed in many ways largely because it is fails to understand foundational topics in the area of how text layers, shape layers and sourceRectAtTime() works. While the technique works well for its specific use-case, chances are you won't be creating buttons in After Effects and expect to get paid - chances are you will want to be able to craft solutions that clients will pay for and hence, this tutorial has actually set up on the path of destruction because it's foundationally wrong/weak/sad/funny - all at the same time. 

I'm not certain who's done a proper tutorial in this area but forums by and large are a better place to get technical questions answered correctly than Youtube. I will qualify by stating that basic-to-intermediate level technical stuff can be safely consumed on social media such as Youtube - the issue is not many know if a topic is in this starting-off category or something more esoteric. 

If you are keen to craft Motion Graphics Design Systems and truly master Texts and Shape Layers, take a look at my LIVE, online course, Even if you don't want to learn or have the time to do so - I do list out topics covered in the course and you can take note of these topics, to search them out on your own. 

https://www.broadcastgems.com/consulting-training

And no courses planned for the rest of this year - so, this isn't a sale pitch - so excuse YOU if you think it's one.  😉

In your case, the issue is within the shape layer itself.
In short, your shape group's Position is offset by 75.4, -3.2, which I'll wager is exactly the amount your box is visually offset from the text. Zero that out, and this should look the way you're intending. 

Context:

Shape layers have multiple places to adjust the Transform properties for a given object. If you're only using a single object per layer, this may feel like complicated overkill, but it functions like this because you can actually have multiple paths or groups within a single shape layer. 

When you draw a single rectangle shape layer, it creates a shape group (1), which contains a parametric rectangle path. That path has its own Position property (2). The entire group (which can contain multiple paths) has an entire set of Transform properties, including an Anchor Point and Position (3). This is where yours is offset. All of this exists as a layer, so it has Layer Transforms, which of course have Anchor Point and Position (4) like any other layer would.