This article will address the following error:
Unique metrics can only be requested in 1/7/28 day ranges.
Cause
Fields in Facebook Insights that count individual users are categorized as "unique." These metrics can only be summed accurately in 1 day, 7 day, or 28 day periods. Reach is the most common "unique" metric, but there are many others.
Solution
Adjust the date range to include 1, 7, or 28 days.

The Facebook Insights API does not report the most recent 48 hours of data.