This issue occurs when people try to log into their dashboards from a mobile device. The first time you log into your dashboard, there's a popup with a checkbox and a button that indicate acceptance of the Terms of Service. You'll need to log in from a laptop or desktop in order to see and click on that checkbox. Afterwards, you can log into your dashboard from a mobile device.
On a related note, entering a device key to use the app constitutes acceptance of the Terms of Service. This means you can start using the app before you ever log into your dashboard. When you do get to the dashboard, all your review requests will be logged under the "Reporting" tab.