The Character Movement Component has a property called Walkable Floor Angle. It defaults to around 45 degrees. So, when the Character attempts to move across a surface that is angled higher than this, the Character will not be able to climb the incline or will lose their footing and slide down it.
Depending on the editor you are in, or if you are looking at an Actor in the level, you will find a property, or expandable grouping of properties, prefixed with Override Walkable Slope. Here you can set the Walkable Slope Behavior and Walkable Slope Angle.