3813826 - Why did I receive an error message "You must identify an approver before the expense report moves on to the next workflow step"? (Default Approver)

I am an Expense Approver and am unable to approve an expense report. The error message displayed while I am trying to approve the report is, "You must identify an approver before the expense report moves on to the next workflow step. Type the first letter(s) of the search criteria (approver's name, email address, etc.). Then, select from the resulting list." The next step in the workflow is the Default Approver / Manager Approval step. 

The Expense Approver is unable to approve an expense report. The error message displayed while trying to approve the report: 'You must identify an approver before the expense report moves on to the next workflow step' , KBA , CON-EXP-ADM-WFL-WL , Workflow , Problem