You must be able to generate an SSH key in order to be able to add it to the Control Panel.
To access the SFTP Management in Control Panel, go to:
Experience Cloud Home > Solution picker: Campaign > Control Panel card > SFTP card
Directly from the URL: http://experience.adobe.com/#/controlpanel/sftp
Learn how to generate an SSH key using a terminal and how to store the public version of the key in the Control Panel.