Well first things first, I am guessing you got this from the Ubuntu Repo? It appears to have been compiled without support for LV2 UIs, or at least the UIs used by those LV2s. I would recommend utilizing a package from this site, or you will have to compile your own.
In fact libsuil-dev was missing: I installed it now from synaptic.
Do you advise me to compile Ardour version 7.1.0? Can this work on Ubuntu 18.04?
Can I keep version 6.9.0 or are there conflict issues?
You can compile yourself, or download the pre-built software from ardour.org.
You can have multiple versions of the pre-built software installed, because each version is installed into a separate directory in /opt. If you compile yourself it is possible, but you may need to check your configure settings to make sure each version has a unique name. I think it should be default.