The last location you'll find yourself in Shadow of the Tomb Raider is Mission of San Juan. There is only one challenge available here and it covers the entire area of the location.
Raise the Flags
To start the Raise the flags challenge, you must hang the first flag. The challenge area is active throughout the entire Mission of San Juan area.
You will find 5 flags on the Mission of San Juan map. They are not pulled up. To raise them you have to cut the rope with a knife. This will release the flags and raise them to the top.
To complete the Raise the Flags challenge, you must cut the five ropes that hold the flags in the city of San Juan. After rising the 5 flags you will end the challenge.