I am having trouble installing custom fonts, I have LibreOffice 6.4.1.2 installed, and my operating system is Zorin OS Core 15.2. I don’t know what I am doing wrong, I have tried different font file types, and placing the font files in the font folder in various locations on my computer.
Your question has nothing to do with LibreOffice and, thus, with this site. Refer to your OS manual or ask in a relevant forum. Wherever you ask, however, ask good questions. In particular, you say:
placing the font files in the font folder
What is the location that you refer to as the font folder? What are the files that you refer to as the font files?