It means that the CPU cooler might need a new mounting bracket for that specific socket.
It should come with it by now.
Ah ok thanks for the reply!
As ImperiousBattlestar mentioned above and according to Noctua, AM4 mounting is included since 2019. (for the most of the Noctua coolers)
That applies to the NH-U9s as well.
You can even check yourself if the cooler in question is compatible by now, by following the specifications on the packaging. If you see AM4, it's compatible. :)
Oh it comes with it? ok thanks for the reply!
Yes that particular cooler is compatible with an AM4 socket board as we speak.