Robert Cohn's repetition of "I can't do it" while he is sleeping could mean several things. Cohn has been trying to convince Jake to travel with him to South America, but Jake has refused to consider undertaking this trip with his friend. Cohn tells Jake earlier that "I'll be too old before I can ever do it." When Jake suggest they travel to British East Africa instead, Cohn remarks, "No, I wouldn't like that." Cohn has also been experiencing writer's block toward his second book--hence the urgency to travel to South America for new material. Lastly, Cohn has been having second thoughts about his new fiance, Frances, and he has already made plans to send her to England for a short time.
In Chapter II of the book, Robert Cohen is sleeping in the office. When Jake Barnes goes to wake him up, Cohen says "I can't do it."
To me, this statement is meant to refer to being content with one's current surroundings and life. Jake and Robert have just spent the whole chapter talking about why Robert is unhappy and why he can't just be content to stay in Paris.
Since this is what the whole chapter has been about, and since this inability to find happiness is one of Robert's main traits, that is what I think he is referring to when he talks in his sleep.
for me, the line "i can't do it" stands for i can't fall in love with Brett, even if in chapter 2 cohn doesn't know Brett yet, but Hemingway knew that, and made it clear to him in his dream, if you read the whole novel you will understand that Cohn was afraid that Jake would be upset with him for fallin in love with Brett as Cohn knows that Jake is madely in love with Brett.
that's from my point of view