I was able to make an adult cat and it gave me the same message. So I hit the accept button (√) and then it took me to the household screen you normally get. I chose the name of the household and it loaded me into the game in edit town and from there I got to choose which household I joined.
Maybe I'm doing something wrong because when I hit the (√) it takes me to the framed picture that you normally get when you finish a household but at that point the (√) is still grey and my only option is to hit the (X) to cancel.
I must be missing something.
Edit. Ok, I figured it out. *slaps forehead* you have to actually add the last name of the family you want the pet to be in!