Hi Anjana,
Uploading Items and BOM simultaneously is not possible. First you need to upload all Items and then need to import BOM.
If you are using 9.1 version, You can easily import Item master from excel, Just create an excel with all item code and description and just import using Import from excel and next you can import BOM through DTW.
But first Item code should be in system this is a Thumb rule.
Thanks,
Harshal