Mail Merge was in tools yesterday, but then I got out to update some addresses. When I returned, Mail Merge wasn’t under Tools and I can’t find anywhere.
What version of LibreOffice do you have?
Because the question was tagged “common”, it sounds like you may not realize how different components work in LibreOffice: Writer for word processing (.odt files), Calc for spreadsheets (.ods files), Base for databases (.odb files) and more.
The addresses for the mail merge may be stored in Calc or Base, but then go to Writer for the Mail Merge Wizard. For example, File → New → Text Document.
If for some reason it is difficult to find the menu option, or perhaps it has moved, then you could use the toolbar button. Documentation for the toolbar button: Mail Merge Wizard - LibreOffice Help.
Complete mail merge documentation: https://wiki.documentfoundation.org/images/3/3c/WG4211-UsingMailMerge.pdf.