The part reject button is not visible on the workcenter tablet view,
Utilization view doesn't display part count, which is what rejecting parts and jobs use as a key metric. Once you change to performance view, or "Parts Goal" you get the reject part button.
To change the tablet to the "Performance" view, click the gear in the upper right corner, and select Tablets from the drop down.
Find the tablet you'd like to change the view for, and click on it. In the settings window, set the Workcenter View, to Performance. Then click the Save Changes button.
For the Performance view to function correctly, you will want to configure a job to run on that machine, otherwise, MachineMetrics won't have any performance standards to compare the machine's activity against. If you'd rather not use any jobs, you might consider staying on the Utilization view and rejecting parts via the web app.