Hi Sebastien,
I know I responded directly to your query, but I should also include a response here in the forum....
The Support APIs, including SN2Info API, are included in the Smart Net Total Care (SNTC) service offer and Partner Support Service (PSS) offer for partners. Since the APIs are available only through the two offers, there is an on-boarding process required before you can use them (which is why you were getting the Not Authorized message).
Details on the on-boarding process are at Support APIs DevNet site: https://developer.cisco.com/site/support-apis/. There you'll find links to Getting Started as a Smart Net Total Care customer and a Partner Support Service partner.
Regards,
Keith