launch filed when snapcraft. How can I use the downloaded .img file rather than download online by terminal?

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Hi, I follow the tutorial http://wiki.ros.org/ROS/Tutorials/Pac... to learn ROS. But an error occured :"launch failed: failed to download from 'https://cloud-images.ubuntu.com/minim...': Network timeout". I know this is a problem about network, someone else have the same problem , they're advised to reinstall or choose an another time to download. But these method is not work for me. I can download the 'https://cloud-images.ubuntu.com/minim...' by internet not through terminal. How can I use the downloaded file? I can't solve the network problem... My ROS version is melodic, OS: Ubuntu 18.04.4 LTS, kernel:5.3.0-61-generic