Other posts have suggested that the ability to type with diacritic markings is set in the OS and not on Libre Office. However, I’m able to type easily with diacritics in all other applications on my Mac except Libre Office. Why is this?
The solution to use the OS based input method by pressing a key to bring up a pop-up menu where you can select the letter with correct diacritics doesn’t seem to work in LibreOffice, see for instance this bug report: https://bugs.documentfoundation.org/show_bug.cgi?id=49967. Failing that, you might try a keyboard configuration with so-called dead keys that allows you to enter, for instance ë by pressing "+e and é by pressing '+e. I don’t know if that will work for you, as I don’t know what language you work in.
On this board, users, some of them actually developers, help other users get on with LibreOffice. You help them help you by giving as much information as possible. Like, in your case: what methods you have tried, how other applications apparently handle it, what language you need it for. Sometimes the version of LibreOffice and/or OS is relevant as well.
I need to type in Italian and Arabic, and then also a couple phonetic alphabets. So creating a bunch of macros for individual characters isn’t an ideal solution for me. And I really don’t know what else to do. I can’t set the keyboard to Italian or Arabic, because these languages on a Mac depend on being able to hold a key down and use the popup box.
That’s a long time I haven’t worked under MacOS, but as far as I remember, you can preselect (tick) several keyboard layouts in the “international” menu in the top bar and rotate between them with ⌘+Space or something similar. This should solve the Italian and Arabic entry.
For the phonetic symbols, I fear there is no other possibility than entering their Unicode encoding with “U+xxxx” (without quotes) followed by Alt+X, where xxxx is the hexadecimal encoding. The IPA block starts at U+0250.