First thing - go back and use [code]...[/code] tags around the outputs in your initial post so we can read them.
Disk /dev/mapper/kubuntu-root doesn't contain a valid partition table
I do not understand why I have the message about not containing a valid
partition table. Can I do something about it ?
This is an artefact of LVM - don't worry about it; nothing is wrong, nothing to fix.
tldp.org has a how-to for LVM - it discusses how to resize lv's safely. Redhat/Fedora also offer system-config-lvm as a GUI front-end - other distros may have similar.
There is no obvious need to reduce the extended, you could shrink the lv and simply use another logical(s) for your new partition(s). I find that a more flexible solution.