How to change the font guys?

Hi! Ive been using it for few weeks now. But i dont understand about the custom font. I upload few custom fonts that i like with correct type of file. Its uploaded and i set it as the font for the book. But when i close the app and open it again, its showing red pop up saying "unable to download font : font-name", or something like unavailable to download, or failed to download.

I try to delete all the custom fonts, but the pop up keep came out when i open the app again. It took a while to stop. So i had to reinstall it. Is there a way to fix that? Or maybe a setting to prevent that pop up happen?