The owner of the shared mailbox you are trying to access has not given you the permissions to send email on its behalf. To ...

The owner of the shared mailbox you are trying to access has not given you the permissions to send email on its behalf. To fix this issue ask the mailbox owner to grant "Full Access" and "Send on Behalf" permissions to you. For more information on shared mailbox permissions, please see the article  Shared mailboxes in Exchange Online.