Hi guys! 

I am in task of adding/assigning topic/s to specific knowledge article and found out this chatter REST API Topic and Article Assignments 
and looks like this is the one I need? 

Okay, so upon testing and following the steps on this documentation, I encountered an error Response error Invalid value for article topic operation type: AssignTopicsToArticle error 400

Here is my actual url 

https://marvingf-dev-ed.my.salesforce.com/services/data/v40.0/connect/communities/0DB7F000000fy3WWAQ/topics/data-category-groups/MarvinGroup/data-categories/Marv Category/articles

https://marvingf-dev-ed.my.salesforce.com/services/data/v40.0/connect/communities/0DB7F000000fy3WWAQ/topics/data-category-groups/MarvinGroup/data-categories/Marv Category/articles

and this is my request body
Header: Content-Type = application/json

{ "operation": "AssignTopicsToArticle", "topicNames": ["API", "Chatter REST API", "ConnectApi"] }

Error Response:

[ { "errorCode": "POST_BODY_PARSE_ERROR", "message": "Invalid value for article topic operation type: AssignTopicsToArticle" } ]

I already set up the following; 

* community website
* created a data categories
* Enabled Topics for object

Maybe I forgot to config something? or Is there an alternative ways to assign a topics for specific article using REST API? thanks!

Regards,
Marvin