I have created a spreadsheet with my address data for my labels. I created a registered database based on this spreadsheet. I did File->New->Labels using Avery A$ type of L7165 and setup a label using the database's table fields. I then did View->Sources and saw the address data above the sheet of labels. However, when I did Tools->MailMerge, on the 4th screen it greyed out next preventing me to go to the 5th and 6th screens and therefore the labels page was not populated with the address data. I'm using LO 5.4.3.2 under Ubuntu 16.04. This worked OK a year ago under a previous version of LO also under Ubuntu 16.04. Is there a workaround?
Just checking: you used instructions like these, right? https://www.youtube.com/watch?v=7n8Psg8yxPE
This doesn't look like bug report (explained why feature doesn't work) but help request (not sure how to use feature). For that purpose, there's ask.libreoffice.org. Please bear in mind there are many bugs to triage and improve LO so no time for help here. What you can report is a need to improve documentation, if you tried to used it and found out you don't get instructions. Like, what goes first and what next, Mail merge, database registration/sources, Labels.
Well, it looked like a bug report to me as the description mentioned it working before. Anyway, there is bug 114369 for simplifying the whole process.