Hi Andrei,

> Michael, some times I meet this problen I can not extrude with caps or make planar what it can be?

In order to form a planar end cap, you need to have a closed planar curve. If it won't generate a planar surface then that usually means it's either not planar or not closed.

In your case here it's because it's not closed, you can tell this by selecting the curve and looking at the object type indicator in the upper-right corner of the properties panel:



Notice how it says just plain "Curve" there as the object type? If it was closed it would say "Closed crv" instead.


In this case the problem is in this area here:




The ends overshoot past each other a little bit instead of ending at the same spot, here I've moved one of the ends slightly to the side to show it more easily:




- Michael