Colliders
Make sure that the actual model / mesh of your character has a mesh collider applied to it and that the convex box is checked. Next make sure that your ground / whatever your character lands on has a collider attached to it whether this collider is of type mesh, box or really any collider it shouldn't matter but I'd recommend trying a cube with a box collider first. Also make sure that all of your mesh colliders actually have a mesh applied to them.
For your character controller double check that the size of it matches your characters model in both width and height. Apply the actual Character Controller to the Parent game object in your case named Player and apply the mesh collider to your actual character.
Some Images of how it should look when correctly set up
How the parent "Player" should be set up simply
How the actual character should be set up simply
How your ground should be set up simply