In Assassin's Creed Valhalla, you can find cairns that will give you extra points. This page of our guide has the locations of cairns in Eurvicscire.
You will find the cairn in the western part of the location.
It is not an easy task. You have to arrange them so that they go upright. This way you will reach the determined height. Move the stones so that they lean on each other and won't fall.
This will give you an extra skill point.
If you find yourself in a situation where you can't even put two stones on each other, just reset the game. There is a good chance that something has bugged and you will not be able to build the cairn. Try to reset the game to resume the task.