Are there points of no return or lockout for side content in Red Dead Redemption 2

Duels works by holding down R2 slightly to fill the 'dead eye' meter, while this is happening you need to watch your opponent to see what they are doing as they may not draw straight away, dive out the way, pull out a knife or do any number of unexpected things.

The longer you slightly hold R2 down, the more your 'dead eye' bar will fill and the longer your will have to aim. To draw your weapon and begin aiming you need to push R2 all the way down then release the button. When exactly is the best time to do this will vary wildly in every duel. You just need to watch your opponent and draw when you have enough 'dead eye' time to hit the target you want.

Apart from the actions of your opponent, you also need to consider what you want to aim at once you have drawn. If you don't want to shoot to kill you may require a longer time to aim.

Once you have lined up your shot press R2 again to fire.

If you have waited too long to draw and you don't have time to aim before the opponent shots you, then you can press R2 rapidly again after you have drawn. This usually results in a body shot to your opponent, provided they haven't moved out the way, and will usually kill them.

There are some areas like your camp where movement speed is restricted to a rather slow walk. I suspect you were in such an area. Tapping X should work usually, both on foot and on your horse.