While it is true, okay generally translates to theek hain. Theek, depending on the context, also translates to correct. theek aur galat means right and wrong. But theek hain, always translates to okay.
No, "ठिक् है, धन्यबाद(thik hai, dhanyabaad) is right ,duolingo's answer is correct right. It is not called -"thik, dhanyabaad" in hindi. It is called -"thik hai dhanyabaad" so this answer is become ठिक् है, धन्यबाद