Service Desk Approve / Decline

Approvals is displayed on the card and on a tab called Approvals

On the Card 

If the issue is awaiting a JIRA service desk approval, then the option to Approve / Decline will appear in the quick links section. See Quick links (formerly action menu)

• Approve - will perform the approve action as specified in the workflow.
• Decline - will perform the decline action as specified in the workflow.

Enable approvers field

Enable the approvers customfield in the Inbox global settings and make sure the events are checked to receive notifications.

To configure approvals follow the guide Atlassian is providing: https://confluence.atlassian.com/adminjiraserver/configuring-jira-service-desk-approvals-938847527.html