If you are a newbie and play on mobile and don't know where you can find an argentavis i just want you to know that there are many on an island that's on the left side of the map, right under the snow biome.

To put one to sleep i recommend to use dinosaur gate frames to make a cage and lure it inside so that it can't attack you while you're using your bow or crossbow to put the Argi to sleep.