Hi Sanjeev,
Please firstly check whether your system has already the following notes, and also check the KBAs for some more information of text in SUS.
Note:
1906435 - Message to purchasers is not displayed
1980408 - Message to purchaser is not visible in the order repsonse
KBA:
1760790 - Overview: attachment and text support in SUS
1729627 - Message to Purchaser in SUS confirmation is not transferred to ECC
1957237 - Message from/to Purchaser is Not Displayed in Communication and Collaboration o
Hope they could be of some helps.
Regards,
Wendy