After 3 days of arduous attempts, during which the virtual machine crashed once, it was finally fixed today.
The beginning is to install FFMPEG according to the blogger's article:
(1 message) Install FFmpeg on Ubuntu 18.04_Pretend to be bad Qianqian Jun's blog-CSDN blog_ubuntu18 install ffmpeg 
https://blog.csdn.net/qq_38863413/article/details/105017775 After the installation is complete, you can pass the command Run ffplay to play the video in MP4 format, but if you directly click on the video, the video still cannot be played. Through continuous search for information, I found that because Ubuntu has protection for some formats of video, it can be solved through the help page of ubuntu:
Restricted Formats - Community Help Wiki (ubuntu.com) https://help.ubuntu.com/community/RestrictedFormats In addition, you need to continue to load codec packages commonly used by video players. There are often mistakes in this step, because when using sudo apt-get install, it is always unsuccessful, so the software installation source is replaced with a domestic mirror. Thanks to the blogger for the article:
Ubuntu 20.04 replaces the domestic source - hacker-judgment - blog garden (cnblogs.com) https://www.cnblogs.com/gaojia-hackerone/p/15202230.html and then follow the blogger's tutorial to install the codec software Bag:
(1 message) Solve the problem that MP4 files cannot be played in Ubuntu 18.04 . , mainly in accordance with two:
sudo apt-get update gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly