Package operation failed

Support & Help Requests
1

When I run software update I get

     package operation failed

The updates downloaded ok.

The error window disappeared so fast, I could not write down errors.

I ran sudo apt-get dist-upgrade

  Errors were encountered while processing:
  /var/cache/apt/archives/apt_1.6.6ubuntu0.1_amd64.deb 
  E: Sub-process /usr/bin/dpkg returned an error code (1)
2

I have run

   sudo apt-get dist-upgrade


 Preparing to unpack .../apt_1.6.6ubuntu0.1_amd64.deb ...
 Unpacking apt (1.6.6ubuntu0.1) over (1.6.6) ...
 dpkg: error processing archive /var/cache/apt/archives/apt_1.6.6ubuntu0.1_amd64.deb (--unpack):
 unable to make backup link of './lib/systemd/system/apt-daily.timer' before installing new version:           Operation not permitted
   dpkg-deb: error: paste subprocess was killed by signal (Broken pipe)
  Errors were encountered while processing:
 /var/cache/apt/archives/apt_1.6.6ubuntu0.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
3

Months ago I added the immutable attribute to apt-daily.timer.

When I removed it, software updater started working. ?