The /var/log/dist-upgrade/main.log file you've attached indicates there was a
failure to contact the mirror the the Ubuntu archive which you are using.
2019-05-02 07:02:25,535 DEBUG examining: 'deb http://mirrors.aliyun.com/ubuntu/
cosmic main restricted multiverse'
2019-05-02 07:02:25,536 DEBUG verifySourcesListEntry: deb
http://mirrors.aliyun.com/ubuntu/ disco main restricted multiverse
2019-05-02 07:02:25,536 DEBUG url_downloadable:
http://mirrors.aliyun.com/ubuntu//dists/disco/Release
2019-05-02 07:02:25,536 DEBUG s='http' n='mirrors.aliyun.com'
p='/ubuntu//dists/disco/Release' q='' f=''
2019-05-02 07:02:25,579 DEBUG error from httplib: '<urlopen error [Errno -2]
Name or service not known>'
This indicates and issue with your network at the time the upgrade was
attempted that you'll need to sort out. You might also consider
switching mirrors of the Ubuntu archive.
** Changed in: ubuntu-release-upgrader (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1827307
Title:
Certain files not installed, can't do full upgrade to next ubuntu
edition.
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