Hi Somik,
Don't create any Z/Y variables.
Go to GRFNMW_CONFIGURE Tcode
Select Process Type as SAP_GRAC_AR
Click on Maintain Notification Variables.
Click on Change Mode and add New Entry as "REQUEST_REASON". It prompts a warning as variable should be in customer namespace.
Ignore the warning and Just click enter and then save and add it to transport request.
Now use %REQUEST_REASON% variable in your notification Emails.
Regards,
Madhu.