Hi Rizwana,
In addition to Raman's reply, you can go to RSA2 in your ECC system and check the fields directly. I checked in my system and it is hidden in my system so same way you can check it.
Just to go tcode RSA2 -> Enter your datasource '0MATL_GROUP_TEXT' and click display. Under the fields tab you will get all available fields.
The value 'A' highlighted means that this field is hidden by SAP. Also you can do a F4 help on this to get clear idea.
After you confirm above then you will have to go to RSA6 and change your datasource to unhide this long text field.
Please let us know if there are any questions.
Thanks
Amit