✅ confused by this error

class Player(Vector2 position, Vector2 size) {
    public Vector2 Position { get; } = position;
    public Vector2 Velocity { get; private set; } = Vector2.Zero;
    public Vector2 Size { get; } = size;

    public Rectangle GetRect() {
        return new(Position, Size);
    }

    public void Input() {
        float dt = GetFrameTime();
        Velocity.X = 0.0f;
    }
}