In the Analyze Your Data Over Time module  where it says:

In the default exploration state, as you remember from the previous unit, the query is aggregating on count of rows with no grouping and no filtering. Start a new exploration by going back to the My DTC Sales app and opening the DTC Opportunity dataset. You’re interested in the number of tablets involved, so complete the following steps.
Click Count of Rows to change the measure.
Select Sum in the aggregation functions list.
Select # (number sign) for the measure to see the number of products in your closed opportunities

If I select Sum the only option is amout. Is the trailhead dataset in sync with the directions?

I am currently taking the course Admin Trail Begginer: Getting Started With the Platform, when I proceed to sign up for a DE account I get the following message: "Sorry, we can't enroll you in a Salesforce Developer Edition account because the email address you entered is already in use. To sign up with a different email address, go to https://developer.salesforce.com/form/signup/freetrial.jsp ."

Does this mean I already have a Development Edition Account? If so where would I access it? 
Or should/can I sign up with any email adress fir a DE?