I have this Digital Frame Samsung SPF-105P and RPi3
This frame has an option, that can working on externer display mode - but have only USB port.
How can display raspberry desktop on this frame (set USB as display output) ?
Ui, that is something you’d need to ask the manufacturer. I guess a special driver is required to use it as regular monitor. But just to be sure that it is not detected as screen already, can you show the output of: cat /dev/fb*