Click the Start a free trial link to start a 15-day SaaS trial of our product and join our community as a trial user. If you are an existing customer do not start a free trial.
AppDynamics customers and established members should click the sign in button to authenticate.
Most likely it's just a typo.
The latest version you can view from the Appdynamics downloads is 20.7
Just use 20.7 instead of 20.8 and continue with the installation