I’ve a similar issue like this: LibreOffice not recognizing Courier, except that I’m using LibreOffice on Linux (OpenSuse 42.3, KDE/Plasma 5.8.7).
Just recognized that “Courier” font has “gone”. I’ve some older LO docs where I could use it (shown in italic in font box, displayed correctly in text) , but for new docs it’s not shown as available anymore.
Strange as this is, KDE’s system settings shows “Courier” and “Courier 10 Pitch” as available.
The other question advices to use “Nimbus Mono” as free alternative, same thing: Shown in KDE as available, not shown in LO.
Any hints would be great
Thanks in advance,