Assuming you're talking about the tutorial
here, which tube (try using better terms; It took me a while to realize loop meant that block with a hole in it from the tutorial) are you talking about?
If you're talking about the carved one:
1. Shame on you for carving! It even mentioned it was bad!
2. Okay, there's probably invalid solids that are lurking everywhere. If so, switch to the vertexed one.
If you're talking about the VM'ed one:
You probably messed up on some of the VM'ing. The HL engine doesn't like concave brushes, so you have to stick to convex brushes. If you can't tell if it's convex or concave, imagine putting a extremely stretchable rubber band around it. If it touches all sides, it's fine. If it doesn't, it's most likely concave.
But so we can analyze this problem further, post your compile log.