And here is the NFL Bylaws and Constitution: Clickhere.
Here is our U.S. Constitution -1st amendment:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
And maybe the President should read the bylaws and constitutions for NFL, NBA, WNBA, NHL, etc. as well as the U.S. Constitution. NFL players aren’t mandated to stand for the national anthem by law.