Hmm, I think the answer key is wrong here. Duolingo accepts "He cannot play soccer," but with the following warning:
> You missed a space. He can_not play soccer >
I think this should be reversed. "Cannot" is correct, and Duolingo should accept "can not" and gently remind the user to remove the space. Reported.
For more information, see here ("Don’t use 'can not' when you mean 'cannot.'"):