When you’re hunting a legendary animal, it is rather important being in a silent zone to better tracking it. Unfortunately, this isn’t possible if there are too many NPCs in the area.
Some player wondered if there was a way to hunt legendary animals even when the “You have entered the legendary animal territory, but there is too much activity to track the animal” message appears.
Hunting Legendary Animals when there is too much activity in the area
Unfortunately, there isn’t much you can do. As the message says if there is too much activity you can’t track the animal. The best thing you can do is try to dedicate yourself to something else and go there again later.
Some people also noticed that saving the game and reloading it worked for them. You could also try this way but consider that in this case, it will take some time too (you will have to wait for the game loading time and return to the spawn area of the animal since if an area is too crowded you won’t be able to save too).
This is all you can do. For more interesting Red Dead Redemption 2 guides like this refer to the game’s Wiki Page.