Conversations: No Messaging Service assigned to Conversation
This Conversation does not have an assigned Messaging Service, therefore cannot accept non-chat participants.
Possible Causes
You may be attempting to add a non-Chat participant (e.g. on SMS) to a channel from Programmable Chat. As Chat precedes the Conversations product, these channels first need to be provided a Messaging Service before they can access SMS, WhatsApp, or other channels.
Possible Solutions
Update the Conversation with any existing Messaging Service SID on your account. After that, you will be able to add non-Chat participants and continue the conversation with them involved.