how I can fix "AppStream cache update completed, but some metadata was ignored due to errors."?"

The bug has been fixed in appstream package version 0.10.1, but the version that gets installed in Ubuntu 16.04 is 0.9.4 by default. Ubuntu 16.10 automatically receives the update as it's provided in its standard repositories.

The updated package version with the bugfix for Ubuntu 16.04 however is only provided through the xenial-backports repository, which has a lower priority than the others. This means it won't upgrade to this version unless you manually specify it.

To enable the xenial-backports repository in first place, follow How do I enable the "backports" repository? (but only enable the repository, you don't need to modify the repo priority).

After that, upgrade to the version from xenial-backports by typing:

sudo apt install appstream/xenial-backports

Now your appstream package should be upgraded to version 0.10.1.

$ appstreamcli --version
AppStream CLI tool version: 0.10.1

Now force refreshing the cache data, you should no longer see any warning:

$ sudo appstreamcli refresh --force
AppStream cache update completed successfully.

See here, fix currently in progress - https://bugs.launchpad.net/ubuntu/+source/appstream/+bug/1644498

The bugs has been fixed per 1 December, in order to get rid the bugs, you have to enable proposed repository under developer options on Software & Updates.

• Open dash then type "Software & Updates".
• Navigate to "Developer Options" tab.
• Enable "Pre-release updates (xenial-proposed)" checkbox.
• Changing repository setting required admin password, fill it on pop up.
• Close the window then, click Reload to apply changed source (Equivalent to apt-get update).
• Open terminal then try sudo apt-get update command.