I took a look at the file, and it seems both the "two-side" of the square is 3. degree, and I guess they werent properly built with the right number of spans (too few spans for a 3. degree curve).

First, I moved the one curve to snap into the other.

Then I rebuild the curves to 1. degree (straight lines), and a simple "attatch curve" made the job.

EDIT:
It seems that one of the curves are an instance. I'm not sure if the curve is build wrong after all. Do you need this instancing? Does it have to be a 3. degree curve.

I got to simply attatch the curves right away, but it left an open corner, and I did not manage to close the curve, by using the close curve tool.