Hello APARAJITA GUHA,
If you did not create this file, you could be dealing with an older version of a Word document or you may not have write permission. While you have the file open you can strike the F12 key on your keyboard to prompt a “Save As” dialogue and save the file under a new name.
Alternatively you can open the document you have, then copy all of the text and paste the text into a new document. You should then not have any issues saving.
I hope that helps!