Publications#LibreOffice_Writer_Guide
I've just started looking into Master Documents, although I have a
suspicion it would be a 'sledgehammer to crack a walnut'.
After studying the info I felt confident enough to try a test, but
struck a rock almost immediately:
I made a small file with just one line of text, did File/Send/Master
Document, got taken to the filename enter box (filter showed "ODF
Master Document"), entered "testmaster" and hit 'Save'.
Result: Error window saying "Can't create document".
No hint of why not, so not sure what to try next...
(Presumably there is some minimum structure missing, but what?!)