Cannot add board link to Arduino IDE?

First off all, make sure the Network connection is in good condition. Hope this post from Arduino helps. If that doesn't work, try replacing the board link from

https://downloadcd.dfrobot.com.cn/FireBeetle/package_DFRobot_index.json to https://downloadcd.dfrobot.com.cn/FireBeetle/package_DFRobot_index.json and reload it.