This page seems to be related with how-to's and what should be where / how to test it:
# https://plantuml.com/graphviz-dot
In short, it seems to expect a file somewhere (/opt/local/bin/dot), but the file is not there.
In general though, if that page is not sufficient, you might want to contact the plugin's vendor as this is not a native app to Confluence, but rather is 3rd party.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.