Mesh collider on a curved tube

I am making a ball game and i need the ball to slide through the tube on the picture. But i cant make the collider match the tube!
And i cant understand the error below…
What am i doing wrong?
Also i used blender to make the tube
and exported it as a .FBX

As dns noted, you have to use convex objects. What I would recommend is using multiple capsule colliders to simulate those old marble-rail type board games, like this. Just make invisible “rail” capsule colliders and put them along the tube edges and see if that works for you.