SFTP license impossible to find how to buy...?

You probably haven't installed your new licence since you added the Advanced FTP option to it.

From Common purchasing, upgrading and licensing questions

I paid for the FTP or USB feature but it doesn't work!

Add-on functions are controlled by the program certificate that you install into your copy of Opus. If you added on these features after your licence was issued you should have received a new program certificate via email. This needs to be installed using the Help / Licence Manager command to enable the new features. If you didn't get the new certificate you can retrieve it from the My Account page on the GP Software website.