I'm trying to select the option Show help center articles when adding a step in the bot builder but it's disabled.
Is it possible to display articles from a non-public help center in the bot builder?
No, the bot builder isn't able to present articles if you restrict your help center visibility. An active, public help center is required and article visibility must be set to Everyone.
For more information on this flow step and an example, see the article: Understanding answer flow step types.