Drink Seer's Mead, then check that the Seer's Sight status is active. Crafting the base or carrying a bottle does not activate sight. By default the effect lasts two minutes, and another bottle cannot be consumed until it expires. Death ends the effect.
Turn toward a supported target within 40 metres and allow up to one second for the next scan. Markers appear through terrain only while the target is in your camera's view. Cyan marks ore, gold marks treasure, and violet marks a crypt entrance.
Detection covers loaded, supported objects. Player-built chests, dropped ore, player-created objects, empty treasure chests and unknown modded prefabs are excluded. A crypt's interior is not scanned from its entrance outside; enter the dungeon to detect its nearby interior loot.
There are at most 64 nearby markers, and very dense areas can exceed the fixed collider scan capacity. Try moving closer to the resource. If the range was changed, check [Mead] SightRadius in the configuration and restart after editing.
Live appearance has not yet been verified inside Valheim. For a reproducible problem, record the game/mod versions, nearby target, other installed mods, configuration and relevant BepInEx/LogOutput.log errors.