Comment 2 for bug 1652757

Your logs show that while you were trying to install chromium, unattended-upgrades was running in the background and that failed, so the failure is unrelated to chromium.
The unattended-upgrades update was also tried on the preceding days and also failed, but unfortunately it is not clear from the logs what the actual problem was.

Please provide (the relevant part of) /var/log/apt/term.log, so that this can be debugged further.

In general, it is a bad idea to report multiple problems in one bug report, because it makes tracking the bugs (and their fixes) much more complicated.
Thus please open a separate against ubuntu-software bug about your second problem.