Shakkei Pavilion is one of the treasure domains in Inazuma. You can find it in the southeastern part of Tatarasuna in Kannazuka. Its location is marked on the map, but you cannot easily access it because it is inside the mountain.
To enter this domain, you have to break a hole in the mountain wall by using a Kamuijima cannon.
However, you need to complete at least the first part of the “Tatara Tales” quest to use these cannons. You get it from Xavier, who is at the entrance of Tatarasuna. Teleport to the waypoint west of the Statue of the Seven and then go down a little to the left, and you can see him standing at the entrance.
Do the Tatara Tales quest up to the part where you need to use cannons to break the barrier preventing anyone from entering Tatarasuna. Then, using the cannon right next to the teleport waypoint to the west of Tatarasuna, fire a cannonball towards the mountain where Shakkei Pavilion is.
Use the map as your guide if you are unsure where to aim.
Once the wall is broken, you can go to the location and activate the domain.