Frank:  The NFL game guide reads "The National Anthem must be played prior to every NFL game, and all players must be on the sideline for the National Anthem"

and continues "While the NFL has no in-game rule on conduct for the anthem, they have commented on the matter. According to Pro Football Talk, the NFL released a statement saying that "players are encouraged but not required to stand during the playing of the National Anthem."

It looks like the National Anthem MUST be played prior to games....that's a stated rule.  However, while players MUST be on the sidelines, the reference to "player behavior during the anthem" is prefaced by the word "should"....that's a very big difference.  It doesn't read like "taking a knee" is a violation of any NFL rule.  Clearly it's contrary to the recommended player behavior, but that's a different thing. 
Gary Crum  here's a link to the rulebook   operations.nfl.com/media/2646/2017-playing-rules.pdf