Hi Amol,
I see very strange situation on your screen. There are two assignments with different parameters on the same date and as result the system check it and create error.
Do i understand correctly you have one open assignment only, but this assignment it's very strange. You could never have created assignment with material 50-100001 (start date 29/04/2011) while the assignment with material 30-100128 was still open because those two materials with different parameters.
In your case you could try to delete a restricted assignment to material 50-100001 mentioned earlier (O4NA>Etit>Delete assignment) and put end date for open assignment.
KR,
Sergey