couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied) [duplicate]
This is a known bug:
https://bugs.launchpad.net/ubuntu/+source/aptitude/+bug/1543280
They suggest running the following command:
sudo chown _apt /var/lib/update-notifier/package-data-downloads/partial/
Give it a try and see if it fixes the issue.