(The following answer was slightly modified on 1/14/19 to clarify a follow-up question, highlighted in blue)
Eriq, an adult amateur player, asks:
Is there a rule whereby, during a penalty kick, a teammate of the kicker unknowingly steps inside the penalty area before the kick was taken and the kicker scores but, because of the teammate, the goal is not counted and an indirect free kick is given to the defending team?
There are no other infringements by the attacking team. When asked, the referee stated that it is a new rule for 2019.
Yes, there is … sort of. And no, it didn’t.
Here’s the “yes, there is” part. Although there are some differences between the International Board’s Laws of the Game and the Rules governing High School (NFHS) and collegiate (NCAA) soccer, US Soccer and the rest of the world follow what the International Board put into its Laws. Law 14 (The Penalty Kick) is very clear regarding this scenario. Purely as a matter of Law, it is an offense for any teammate of the attacking team (i.e., the team that is taking the penalty kick) to do any of the following things before the ball is in play on a penalty kick: (a) enter the penalty area, (b) enter the penalty arc, or (c) move closer than 12 yards to the opposing team’s goal line.
The remedy for such a violation is also very clear purely as a matter of Law. Now comes the “sort of” part — if a goal is scored, the goal does not count and the penalty kick is retaken OR, if the goal is not scored, the defending team is given an indirect free kick where the teammate illegally did whichever of these things we just listed above (i.e., where the teammate illegally entered the arc area or penalty area or where the player came closer to the goal line than 12 yards). The rule is inflexible as to the offense – one foot into the penalty area is just as illegal as 15 feet into the penalty area. Nor does it make any difference whether the violation is the reason for the failure to score a goal – one step into the penalty area or into the penalty arc (which are the two most common ways to commit this offense) but the kicker kicks the ball 50 feet over the top of the goal’s crossbar does not remove the offense. We might note that these limitations on the kicking team apply equally to the defending team: the only difference is that the punishments are different (if a goal is scored, it counts; if it is not scored, the penalty kick is retaken).
So, in your scenario, the teammate of the kicker violated Law 14. However, the restart was incorrect because Law 14 states that the goal is canceled but the attacking team is given a retake of the penalty kick.
Now comes the “no, it didn’t” part. This is not a “new rule for 2019” – in point of fact, the described penalty if a teammate of the kicker violates Law 14 and a goal is not scored went into effect in 2005 but the restart which should have been given in your scenario (cancel goal and order a retake) has been in Law 14 for as long as we have been officiating (1984).