If this template helps then use it. If not then just delete and start from scratch.
OS (Win10): PsychoPy version (it shows 2020.2.10, but I adapted it to 21.1.4, since my program is programed at a 21.1.4 psychopy on another computer): What specifically went wrong?:
everytime when I tried to start the program, it showed key error: fonts. But I have never confronted this problem in the original computer.
Wakefield is right, in 2021.1 we added the ability to use fonts from Google Fonts without having to install them locally - the way it does this is to grab a .ttf file from Google and store it in a folder, the location of this folder is stored in prefs. However, this means that when fontmanager looks for fonts, it doesn’t just check which fonts are installed, it also checks this folder - but if you’re running as 2020.2, there won’t be any reference to this folder’s location in prefs, so you get this error.