The correct translation should be "I cannot," not "I can not."
According to this source, http://www.write.com/writing-guides/general-writing/word-choice/cannot-versus-can-not/, "Technically speaking, the two spellings are interchangeable; however, modern convention and preference use the spelling 'cannot.' Most modern word-processing software even highlights the use of 'can not' as grammatically incorrect. In addition, specific style guides may give the distinction for this preference as well."
As long as Duolingo continues to write "can not" instead of "cannot," the site will look like it is out of touch with contemporary written English.