If you are not able to see CDNUR section/Sales Return in GST Portal, please consider below points:
1. You must Set Off your Sales Voucher for which you are Creating a Sale Return.
2. Check the type of Sale (B2CS or B2CL) for which you are creating Sales Return.
3. If you found your voucher as B2CS then you cannot upload that Sale Return on GST offline tool as GST offline tool does not support CDNUR for B2CS sales.
4. So it is suggested to Net-Off your Sale Return from your B2CS sale voucher.
For more information you can contact GST Portal for the same.