Expression error on the position of the anchor point

Roland, My expression (and preset) is designed to put the Anchor Point of a text layer in the bottom left corner of the text layer. That's what the OP was trying to do. His expression clearly points to thisLayer.

When I want to line up a shape layer with the text layer as you suggest he might want to do, I always snap the anchor point to the bottom of left corner of the text area. The [a.width, a.height] expression will only give you the size of the text area and that is useful for sizing the rectangle, but you need anchor point and position of the text layer to get the box to stick to the text. 

When I am creating a shape layer to go behind text I always use direct references in my expressions because the shape layer will usually be below the text layer. I use an expression to center the anchor point in the text area more often than the one I shared that puts the anchor point at the bottom left. Then it never matters how the text is justified.

Here are my two most used animation presets for creating MOGRT files:

https://www.dropbox.com/s/5cxexyecurx9css/magic%20text%20box.ffx?dl=0

and:

https://www.dropbox.com/s/zs8k2643qfht86g/center%20anchor%20point.ffx?dl=0

The center anchor point preset uses this expression:

txtLyr = thisLayer.sourceRectAtTime();
hlfHt = txtLyr.height/2;
top = txtLyr.top;
cntr = txtLyr.left + txtLyr.width/2;
x = cntr;
y = top + hlfHt;

[x,y]

The Magic Text Box preset uses these expressions and adds a slider control for padding:

// Rectangle size
txt = thisComp.layer(index - 1);
s = txt.sourceRectAtTime();
pad = effect("Padding")("Slider");
[s.width, s.height] + [pad, pad]

//Transform Rectangle Anchor Point
[0, 0]

//Transform Rectangle Position
[0, 0]

//Transform Anchor Point
[0, 0]

//Transform Position
txt = thisComp.layer(index -1).position

All you have to do is type anything in a new text layer, apply the Center Anchor Point preset, deselect the layer and double click the Magic Text Box expression to add a new shape layer that will snap into position under the text as soon as you move it below the text layer. Takes about 10 seconds.