This is the Dialogflow limitation. Dialogflow will only accept the message - text event. But the DateTimePicker Action will return the Postback event. So dialogflow will reject to read this event from LINE Messaging API.
My recommanded workaround is to develop a datetime picker with JS and HTML and use LIFF (
liff.sendMessage()) to send the data back to LINE with a Normal Text Message Type.
Many thanks Siratee! Much appreciated. I'll take a look at your suggestion.