That message is from Facebook, informing you that you have reached your cap for friend requests today. Typically, you must wait until the next day to send more friend requests. This is a measure Facebook has recently implemented for every account and depends on the overall activity associated with the user's account.
Some accounts can send 50 requests daily, while others can send 100 to 200. Unfortunately, there is no way to determine why Facebook tends to limit some accounts more than others. The best course of action is to wait at least 12 hours or until the next day to send friend requests to more users until you reach the daily cap again.