Should the "X" stay put or move to the purple square, which is lined up with the Sign Up Form. Does it matter?
The error banner is sized to the overall dimensions of the content boundaries. So the 'x' is in a perfectly fine place based on the static alignments within the page.
The real answer to the issue is: The error message is in the wrong place.
For multiple reasons, one being the Gestalt Law of Proximity, the error message should be a part of the "Join for Free"
div itself. Details should also be directly associated with the fields in error.