Research is crucial to completing this quest. In Solaceon Ruins located in Crimson Mirelands you have to meet Calaba here you will start with Arezu’s predicament.
Pokemon Legends Arceus: Locating Arezu in the Crimson Mirelands
When you have Ursaluna, it will receive signals about where Arezu is. This is because Ursaluna is able to smell Arezu. If you don’t have an Ursaluna yet, you may try to evolve your Ursaring.
Urslana will receive signals in multiple colors but you need to follow the blue signals.
From the starting location, go straight and cross the small wooden bridge.
Then enter the small forest area and when you come across a large rock, climb to the right side of the rock. Or another option is to throw a Pokémon in the rock to break it and pass.
When you pass on the right side of the row with dried woods you will see Arezu.
Now, what could be bothering her?
ALSO READ: Pokemon Legend Arceus: How to Get All 3 Starters | Rowlet, Oshawott, Cyndaquil