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 Hamtunscire.
You will find the cairn by going north of Wincestre.
It is a quite peculiar cairn. You need to place most of the stones vertically to reach the specified height. The screen above shows an exemplary solution.
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.