I am using Libre Office version 5.1. I have an existing LO spreadsheet with column titles as the first row. When this is specified as the data source for the mail merge Address Block step, the “next” button goes grey and nothing happens. The Base application is available so I think that it has been installed. What is wrong here?
It appears you have not set the fields on the original document and have not set Matched Fields... for the Address Block. For a detailed explanation, see Chapter 11 of the LibreOffice Writer guide - Using Mail Merge. (click here for link).