This page of our Assassin's Creed Valhalla River Raids guide explains if you can use a raven during raids. It matters because there are a lot of secrets and resources to collect in these areas.
Synin cannot assist you during raids. This means that it will be inactive during the entire raid. You cannot summon it in any way. It significantly hinders the exploration of large areas. However, there is nothing you can do about it. There is no way to change this or purchase this option.
Odin's Sight works, however. It scours the surrounding area for raw materials, treasures, keys and all sorts of resources. When you use Odin's Sight, you must focus on both the effects displayed on the screen (e.g. yellow or white dots) and the sounds. It will often be the sound that will guide you to your destination, e.g. you will hear the sound of a key hitting the floor or someone struggling with the chest lock.
Odin's Sight can be activated at any time:
On PS4/PS5, you must hold down the analog stick
On a PC, hold down the specified key
On Xbox, you must hold down the analog stick