If when sending a communication the error "Unable to deliver the submission to at least one recipient" is obtained, this response indicates that the submission cannot be made by an alerting person or complainant given that there is no no recipient configured, or this recipient is not active (has not accessed the environment for the first time).
In other words, it is necessary that at least one recipient user has been selected in the main context and that this person has accessed the management environment; otherwise, and for security reasons, the request cannot be sent.
To fix this, you need to add a recipient as a recipient , and they must log in before attempting to forward the communication.