The action for activating a scene is missing from Rule Machine 5.1 and Button Controller. This screenshot shows that the "Activate scene" action is missing from the drop down list:
It's actually there; you just need to look under Adjust Shades/Blinds or Fans to find the Activate Scenes action. (Some history: the name of this "category" used to mention scenes until Room Lighting was introduced and Groups and Scenes was deprecated and this action removed for new rules, along with renaming the category to reflect that; both decisions were eventually un-done, but the name of the category remained even though the action was added back. Room Lighting would still be the preference, and perhaps the desire to de-emphasize Scenes in favor of this is why.)
Room Lighting can be used to create the same outcome as scenes or groups, but I assume that is not what you mean here, so the above answer is not likely to be applicable to your situation.
Thank God I found this.
I just got around to re-writing a rule from RM Legacy to 5.1 and was stumped on how to activate a scene, that name (shades and blinds) is bizarre, why not append scenes to that...