I'm following that guide but it is not working for me.
I see the CanFulfillIntentRequest is from 2018. Is it still in BETA or was removed from Alexa?
I had two problems:
1) ask api does not exists anymore at ask 2.x. So I installed ask 1.7.23 to run the commands.
2) After change skill.json with interface CAN_FULFILL_INTENT_REQUEST I updated my skill and check it with ask api get-skill. It is updated. But the Build / Custom / Interfaces pages is not showing the item CanFulfillIntentRequest to enable it.
Do you know what is happening?