"the lack of closure"? What does it mean?

No, not really. Often it involves something quite imperfect. When used in this way "closure" generally means some satisfactory state of "finality." A conclusion to something that you can live with and which no longer occupies your thoughts--it is no longer "unfinished business" which requires your attention, doubt, or concern. Sometimes it is said that when the bones of a long-missing child are discovered, the parents can achieve "closure," for example.

However, this author seems to be using it in a different way, like "achieving an ultimate goal." In this contect, it may be implying a certain degree of perfection, but that is generally not a requirement. Used in this way, finally finishing the building of a 40 story building that took years to build would be achieving "closure" with respect to the project. But if the job were done defectively, with the contractor getting hit with a lawsuit for defective workmanship, then there would be no "closure" to the project, even though the building was "finished."

That's the way I understand it, anyway.

Excellent!