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Reports may not be available until you use the Refresh Statistics feature. This can be accessed from the admin section under Reports > Sales > Refresh Statistics.
Simply check off all the statics you need to process, then select the time period you want to refresh for (Lifetime or Last Day) from the *actions* drop down in the top-right, and click Submit.
This is particularly important for generating Sales-related reports like coupons. The reports will be blank until the Refresh Statistics feature is run.