Assassin's Creed Valhalla

Before you can obtain 5 Treasure Hoard rewards, you'll first need to find at least 5 Treasure Hoard Maps. In general, there is usually one map per territory in England, plus one in Norway. They are considered artifacts, and are therefore white dots on your map/compass until you get close enough. While you only need to collect 5 for this achievement, you'll need to collect all of the maps for the Completionist All the Way! (30G) achievement. Refer to it for more information on where to find all of the maps. Once you have the maps, you can then locate the treasure reward for each one. The maps themselves give a hint, but they're pretty vague. Instead, you can consult THIS GUIDE that shows the location of each Treasure Hoard reward. As you approach, you should see a glowing light on the ground where you can press  to loot it.
 
Glitch Warning: There are many reports of Treasure Hoard rewards not working. By that, I mean that when you go to the location of where the treasure itself should be (after obtaining the appropriate map), there is nothing there to interact with in order to collect the treasure. Thankfully there are 20 Treasure Maps in the game, and you only need to collect rewards from 5 of them for this achievement, so at least 5 of them should be working. I personally had 6 glitched before I was able to collect 5 for this achievement. So don't give up, and try each one until you get a total of 5 of them.