Click on Notey (or type "n") and see if you are in the middle of an Adventure or a Favor. If you are, then you can't do anything for anyone else until you finish that first.
If you are not in the middle of something else, then it could be a glitch or something and the game needs to be reset after midnight.
I know others have had this same problem and these two reasons have been the most common. You may want to read through some other threads like the Question about the game one for what others have figured out.
~Vicks