Not much information to work with:
A full error message message might help or if an alert the alert window (with the more details option open).
That said I believe I have seen something in the forums and that related to a games anti-cheat (and one blocking the other), but I can't recall if that was network related or conflict only.
However, since you mention network rules, is your firewall set to Private, Trusted network
Obviously Valorant would suggest uninstalling Avast rather than have to investigate the cause of the conflict.