I figured it out. The file association was not correct for .ttf but it was correct for .otf. The ttf files were opening in an image viewer. That's why there was no "install" button. I know, it should have been obvious!

I have downloaded many fonts. They were all "100% free". I have extracted the zip files. I double click the font file and they all say "demo text".

How can I install the fonts like this? The "install" button doesn't appear when it says demo.