You have to be an administrator (OP) for this to work. See our guide on how to make yourself an admin on Minecraft: https://scalacube.com/help/gameservers/minecraft/how-to-become-an-admin-on-your-minecraft-server
Log into your server.
Type in "/gamerule announceAdvancements false".
Player advancement messages are now disabled. Typing the same command with "true" instead of "false" will enable it again.