No, it is not possible to create a legend for map tours. However, this article provides two alternatives to a classic legend; to use a graphic legend or to encode the map layer’s color to the text.
Use a graphic legend
Add an image representing a graphic legend before the map tour to explain the features in the story. Refer to ArcGIS StoryMaps: Add images and videos for instructions and more information.
The image below displays a graphic legend for ‘Private School’ and ‘Government School’.
Encode the map layer’s color to the text
The image below displays two bulleted list text encoded with the color of the ‘Private School’ and ‘Government School’ feature layers.