E28012 — AMBConversationNotFound

No conversation matches the identifier given.

On a send, this means the (from, to) pair names no conversation. Apple issues a customer identifier per brand, so check that from is the business this customer opened a conversation with, and that to is the opaque_user_id from that conversation rather than one from another brand. Your business cannot open a conversation, so a customer who has never messaged you has no identifier to send to. Elsewhere it means no conversation matches the id given.