PS5/XSX - It's not possible to start the game in SSF mode.

PS5/XSX - When creating a new character, I can't select SSF mode, the controller doesn't respond. I tested it on both consoles...

It must first be unlocked by rescuing the scion at least one time before you can enter SSF. It should be stated in the character creation screen.

They should probably remove this restriction, not sure why it was there in the first place.

See the 2.6.0 full patch notes:

"

This mode is available once you have rescued the Scion on your account (which can be done in a previous league, so almost all existing players will already have fulfilled this).

https://www.pathofexile.com/forum/view-thread/1839765