On this page of the Assassin's Creed Valhalla game guide, you will find a detailed walkthrough of the side quest in the Wincestre region: Asser.
Reward for completing the mission: 1200 XP.
In the place indicated on the image above, you will meet a young priest named Asser. The elderly priest who accompanies him is apparently heartily fed up with him and wants God to give Asser the sign and enlightenment he so devoutly asks for.
The task is to illuminate the wooden lectern, and the pious Asser will take this as a sign from God. Go outside and climb the wooden scaffolding.
You will notice heavy bookcases by the windows. Move the one shown in the picture above to let more light into the church, and this will illuminate one of the lecterns, so it triggers Asser's interest. Go downstairs and speak with Asser to complete this quest.