You can copyright the rulebook and the description of the rules.
You can’t copyright the rules themselves.
Like, for a dumb example, if you invented the game of baseball, you could write a book explaining the rules of baseball and how to play, and you could copyright that book and if anybody made unauthorized copies of that book or even just portions of it, you could sue them for copyright infringement. But you could not sue people for copyright infringement just because they were playing baseball without your permission … or writing books about playing baseball, etc.
You can copyright the rulebook and the description of the rules.
You can’t copyright the rules themselves.
Like, for a dumb example, if you invented the game of baseball, you could write a book explaining the rules of baseball and how to play, and you could copyright that book and if anybody made unauthorized copies of that book or even just portions of it, you could sue them for copyright infringement. But you could not sue people for copyright infringement just because they were playing baseball without your permission … or writing books about playing baseball, etc.