The user is performing some tests in Sandbox where they try to approve a sample transaction (e.g. Purchase Order) via email.
The Approve or Reject button is available within the email's body and lets users click the Approve button if they wish to approve the transaction.
Once clicked an email is generated and the user will just need to send it to approve. However, the transaction is still in Pending Approval and does not get to Approved status.
Ensure that the Approval Request Email Recipient field found in Setup > Approval Process Manager > Preferences should contain the same value as the one found in the SuiteApprovals Email Approval field on the Manage Plug-in Implementations page which can be checked in Customization > Plug-ins > Manage Plugins.