Just to double check: you're entering names of google fonts installed on your computer, right? Because you can use any fonts (not just google fonts) but they need to be installed locally.
Huh... that's weird, 'cause the ones that did work, I definitely did not have installed locally, and then they suddenly stopped working.
Otherwise, I had just been using a font manager, but it wasn't in use when I was able to use the fonts that I'd normally have it open to be able to use, and when it is open, they don't work anyway, now.