I have a document that I was previously able to mail merge from multiple times however once I select the address list to merge with it it does not move forward in the wizard to allow the merge to proceed. I have attempted multiple address info files as well as recreating and even starting from a completely fresh writer doc, all yield the same behavior, mail merge is broken on my LibreOffice install. I was having this issue on an earlier stable version number and also have now updated to the latest (5.4.3.2) with no change in the problem.
I saw this thread: Why can't I get past the 'address block' step in the Mail Merge wizard on Ubuntu? it does not help as I am not on Ubuntu.