One of the first things I tend to do when starting my system is looking for updates.
I do this by starting YaST and going to 'Online Updates'.

Not once but frequently I am being prompted by the message that the system is blocked by PackageKit and YaST won't be able to refresh the system repo's.

I could wait for PackageKit to end it's job, but this takes a very long time.

Even 'sudo zypper update' won't work.

On searching the Internet I found this command:

sudo killall -KILL packagekitd

This solves the (that is: my) problem! Now YaST can do its work.

The next option would be: Disable packagekit deamon!

PackageKit can be controlled through systemd like this:

systemctl stop packagekit

systemctl disable packagekit