Hello Stephane,
I’m working on a model started from scratch on 5.1.0 on Windows 10. I was working under 1.4 and switched to the latest version because of this issue. I’m using a LAB diagram. When right clicking on the functional chain in the ‘Project Explorer’, i have the “Add Capella Element” as shown heredunder:
But i don’t have the possibility to create a new diagram (LFCD if i remember well). On an older version i was using (1.2), i can do it.