permission error when installing ttf-mscorefonts-installer/Can't drop privileges as file couldn't be accessed by user '_apt'

The underlying problem

Can't drop privileges for downloading as file '/var/lib/update-notifier/package-data-downloads/partial/webdin32.exe' couldn't be accessed by user '_apt'.

This is indicating that the _apt user doesn't have access to write to the /var/lib/update-notifier/package-data-downloads/partial directory, or to any existing files there. As a consequence, the installation will not be able to extract font files to - in turn - update (or install for the first time), the indicated fonts.

I contend that the package is therefore NOT properly installed, unlike the currently accepted answer (

What should you do?

First, you'll want to verify that you have an _apt user as suggested here with the following command:

getent passwd _apt

If for some reason you do not have the _apt user and need instructions for installing it, please go to the link indicated above.

Next, you will want to assign the directory indicated, as well as all files and subdirectories contained within, to the _apt user. You'll want to retain the privileges associated with root superuser group and consequently the root user account. Following the reassignment of ownership of the directory, we should perform some directory clean-up and then finally ensure that the packages and their dependencies are properly installed.

The commands you need...

...after verifying that the _apt user does indeed exist:

sudo chown -R _apt:root /var/lib/update-notifier/package-data-downloads/partial/
sudo rm /var/lib/update-notifier/package-data-downloads/partial/*.FA‌​ILED
sudo apt install --reinstall update-notifier-common ttf-mscorefonts-installer 

This is the bug #1543280, though a rather harmless bug. The installation is completed successfully. The W: signifies that this is just a warning.

Also note that, This is not related with the package ttf-mscorefonts-installer solely, instead whenever apt access already downloaded files, you'll see this warning. It happens also for offline repositories. I have offline repositories which I use to update packages without Internet and I myself see this always in Xenial (16.04). Not a big problem at all.

For your convenience, I am showing the output of reinstalling xrdp from my local repository.

sudo apt-get --reinstall install xrdp 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 398 not upgraded.
Need to get 0 B/215 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 file:/media/Linux/linux_debs_repo/xenial_new  xrdp 0.6.1-2 [215 kB]
(Reading database ... 612447 files and directories currently installed.)
Preparing to unpack ..././debs/xrdp_0.6.1-2_amd64.deb ...
Unpacking xrdp (0.6.1-2) over (0.6.1-2) ...
Processing triggers for systemd (229-4ubuntu6) ...
Processing triggers for ureadahead (0.100.0-19) ...
ureadahead will be reprofiled on next reboot
Processing triggers for man-db (2.7.5-1) ...
Setting up xrdp (0.6.1-2) ...
N: Can't drop privileges for downloading as file '/media/Linux/linux_debs_repo/xenial_new/./debs/xrdp_0.6.1-2_amd64.deb' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

Check this question about Apt's status in Unix and Linux SE apt-get update exit status

Another related bug is #1570141

Try this it worked for me.

sudo dpkg -i ttf-mscorefonts-installer_3.6_all.deb