YouTube Reach (Unique Viewers) is only available for the preset ranges of Last 7 Days and Last 28 Days because those are the only periods where YouTube provides a de-duplicated total.
For any other custom date range, it’s not possible to calculate an accurate total without double-counting viewers who return across multiple days. Adding up daily reach values would inflate the number and give you misleading data. To ensure your reporting stays accurate and trustworthy, we only display reach totals when YouTube can guarantee the numbers are properly de-duplicated.