Build a Travel Approval App – Roll-Up Summary Field verification fails although configuration is correct

I am completing the Trailhead project "Build a Travel Approval App". 

The "Create a Roll-Up Summary Field" step does not verify. 

I have confirmed: 

- Expense Item → Travel Approval is a Master-Detail relationship. 

- Total Expenses is a Roll-Up Summary field. 

- Summary Type = SUM. 

- Field to Aggregate = Amount. 

- Filter Criteria = All records. 

- The field is on the page layout. 

The verifier still returns: 

"Whoops, looks like there was a problem. Please try again." 

Has anyone encountered this verifier issue?