On this page of the Assassin's Creed Valhalla Wrath of the Druids game guide you will find a detailed walkthrough of the Trouble Brewing quest.
By rebuilding the trade post in Athlone, you will gain access to Ale - one of the main luxury goods in the Wrath of the Druids expansion pack.
Reward for completing the mission: 1,700 XP.
Investigate the Trade Post
Once you have cleared the trade post, read the note that lies on the wooden barrel in the location marked in the image above. You will learn where to look for the document you'll need to rebuild this trade post.
Obtain the deed
Head to the location indicated on the world map and use Odin's Sight to locate the the chest containing the document. Eliminate the enemies stationed in the camp, take the document and return to the trade post.
Restore the Trade Post
Return to the trade post in Athlone, interact with the wooden plaque and confirm the construction of the new building. This will complete the Trouble Brewing quest and you will receive 1,700 XP as a reward.