Hi Maurice, and welcome.
The answer is quite simple.
The ‘content’ slot you have selected only allows Entry or Section archetypes, whilst TNM is a cluster archetype. Cluster archetypes are intended to be slotted into Entry archetypes.
Currently we would carry a TNM or other staging archetype within a Problem/daignosis archetype but I have seen a proposed ‘Staging’ archetype from Welsh colleagues that might act as an alternative container.
Add the problem-diagnosis archetype in ‘content’ then add TNM inside that.
It is a newbie ‘issue’ that still catches most of us out from time to time!!