Hi Lawrence,
I realize this is an older post, but it appeared in my "recommended content" list of GeoNet threads while I was responding to another question, so I thought I would post a reply.
> ... my department wanted to make use of the portal to integrate wordpress or create forum so that members of the department can post/share/ask information to the whole department. Then, I came across with a documentation for Portal for ArcGIS which indicates the possibility. ...
"Commercial—Enable forums, blogs, and sharing to social media."
Set advanced portal options—Portal for ArcGIS (10.6) | ArcGIS Enterprise
I can confirm that there is no discussion forum type capability within Portal for ArcGIS. The help documentation you reference above (red text) is an error and incorrect. Thanks for bringing this to our attention.
Hope this helps,