Hi,
Thanks for your question. There's nothing preventing you from choosing (not targeting) a sorcery with the Briefing, even if you cast it on your opponent's turn. However, you are not able to cast the sorcery in this case. The Briefing does not change when you can cast the spell. This is because the Briefing gives you a time frame in which you can cast the card: “this turn”. Since it's giving you a time frame in which you can cast the card, if you choose a sorcery, then you can only cast the sorcery if it's one of your main phases when the stack is empty.
Compare the wording of the Briefing to a card like
Omnispell Adept - it's not giving you a time frame to cast the card, it's instructing you to cast it when the ability resolves, meaning it gets around normal timing restrictions (and why you can use the Adept to cast a sorcery on an opponent's turn).
Thanks,
Nathan Long
Magic Rules NetRep