Hi Amarendra,
There is as such no hard and fast rule that says you can't do it, but since the SAP HR role itself seeks person with previous work experience in HR or as the case with other SAP functional modules, it is better to do it with some exposure of HR real time work. But as it seems you are quite a student as of now, you can try out SIEMENS or Genovate to have the SAP Functional Certification (Cost: Approx 3.75lakhs).
If any further queries, let me know.