I’m using libreoffice writer 5.2.7.2 on debian/ubuntu
I can see “file>export” and “file>export as PDF” but no sign of epub?
Of course, there is none. Epub export was implemented in LO 6.0. Upgrade or use a third-party extension.
https://wiki.documentfoundation.org/EPUB/Converting_ODF_to_EPUB