It appears that the recent UI change for the close vote review dialog introduced a bug when it concerns suggested dupes.
Reproducing in Firefox 78.0.1 (Mac OS). In this dialog there are two tabs, "Question" that shows the question to be closed, and "Duplicate" that shows the suggested duplicate. When the latter tab is active, the "Close" button produces no action though it is not disabled.
Only when on the "Question" tab can you successfully vote to close the question.
There seems to be an uncaught exception raised by JQuery, when called from
SyntaxError: Document.querySelectorAll: '.close-question-link[data-questionid=63380604]' is not a valid selector
Looks like the original question ID may be lost when switching to the "Duplicate" tab.