I don't get the meaning of the sign <<>> and also from which line the error is coming as I have a program of more than 30 lines with many defined functions without using "Module" . Is not there any way to display line numbers in Mathematica so that we can know from which line error is coming?
<< ..>> is called a Skeleton. It is is used to show you an abbreviated version of the actual offending line. You can find it in
Short[(x + y)^30 // Expand]
You can see the skeleton right there in the middle.
The error message probably comes from an illegal construction like:
(a + b)[x_] := 2
SetDelayed::write: Tag Plus in (a+b)[x_] is Protected. >>