Belt or pulley squealing?

More than likely it's the belt itself, over time the tensioners get weak and the belt gets glazed and then it starts to slip. Please never use any form of lube or belt dressing on them, it actually makes it worse because you then need to clean all the parts that the belt drives when you replace the belt. For testing you can use plain water while watching the various pulleys and idlers. If you put water on the belt and the squealing stops and none of the various driven pieces stops then it's the belt. In this case if the belt has never been changed I would change the belt and the tensioner. That one isn't bad to change you just need a ratchet or a serpentine belt tool. Make note of how the belt runs around the pulleys and then rotate the tensioner counterclockwise as shown and remove the belt. Now use some carb cleaner or similar solvent and a brush and clean off all the pulleys in the system. Then remove the tensioner and replace it with a new one. Install the new belt by releasing the tensioner and letting it move once the belt is in place.

https://www.2carpros.com/articles/replace-serpentine-belt