Hello Aishwarya,
What's your SRM scenario, is it PPS. As novated PO error comes when you are trying to do some mass change in or transfer of PO's from one supplier to another. Please have a look to this link.
Mass Change and Novation - Features of SAP SRM Procurement for Public Sector Basis - SAP Library
Also please go through the reply of muthuraman
Change vendor in SRM contract.
Thanks
Gaurav Gautam