why does the cube stop when it hit separate walls

i added for the cube addforce to move forward
and the walls only have meshcollider and rigidbody
i don’t know how to explain here is clip

In future, try and be more specific when describing your problem.

Assuming I understand correctly and the issue is that the cube slows down when touching the walls, there should be a simple fix for that.

Create a physics material and apply it to both the cube and the walls, and make sure the friction is set to 0 on the material.

Side note - Why are you using mesh colliders on rectangular walls? Use the simplest collider available at all times; in this case the box collider.