Do you really want to teach your baby no? :) Seriously, no is a wonderful way for a baby to avoid frustration by enabling them to communicate when they object to something. You can teach no whenever you ask questions to give your baby an option. Say you offer your baby their pacifier, and she does not take it. You can then say, "No - you don't want the pacifier."
No is a sign that is easier to teach in real life rather than using a flash card. You can still use the flash cards for other things like teaching your baby to recognize words when she gets a little older.