Hi Santoshini,

In this scenario you have to check on the following parameters
 
1. Check whether you have created your Sales organisation
properly & saved in Defining part of Enterprise structure .

2.In your Assigning part of enterprise structure first check
whether you have properly assigned your sales organisation
to the Company code of yours .

3.If you have done all the above step properly ,then you
just check your sales organisation would have got and
assigned in some other distribution channel or the standard
0001 in your (Assign distribution channel to sales
organisation screen ) you will find your sales org assigned
with some other distriibution channel select in that DELETE
  & SAVE  then again you can go & position to your
distribution channel & you will get your sales organisation
    & assign & save .


Regards
Shashank

this common problem faced in ecc 6.0 version u need not to 
worry find it manually.