Don't Starve is going away today (replaced by Outlast), yes, but it's not supposed to be gone until midnight or something. And as you can see it's clearly still shown as free for PS+ members. But I'm not allowed to download it? What is this shit? Same problem if I try on on the SEN site.
(Yes, I should have downloaded it a bit earlier, but still. WTF.)
I can't read the language the error message is in, but I'll take your word for it. Usually the games are removed when the PS store updates. I guess it depended on when the store updated in your region.