Hello Hemant,
Approvers and Reviewers must be assigned through SPRO.
Go to Governance, Risk and Compliance -> General Settings -> Workflow -> Maintain Custom Agent Determination Rules. In this customizing activity, you will need to assign a role to the following Business events:
Approver - 0FN_POLICY_APPROVE (standard SAP role is SAP_GRC_SPC_CRS_PLC_APPR)
Reviewer - 0FN_POLICY_REVIEW (standard SAP role is SAP_GRC_SPC_CRS_PLC_REVIEW)
Also, there is a business event for the default approver:
0FN_POLICY_DEFAULT_APPR
Best Regards,
Fernando Bassuino