Hi Abhishek,
This is a pure ABAP question, not a BPC question.
First of all, you defined <lt_acc> as a TABLE therefore <lt_acc>-RATETYPE means field RATETYPE of header of that table. This is not allowed in OO ABAP.
Next, you defined ls_acc as a structure. I can't understand meaning of ASSIGN ls_acc->* TO <lt_acc>.
If you want to get line of <lt_acc> in ls_acc you should write a
loop at <lt_acc> into ls_acc.
...
endloop.
Then you can use ls_acc-RATETYPE inside that loop.
Hope this helps.
Gersh