There are various reasons why your legacy opportunity reports may show incomplete or no data. A currency type - USD, AUD, EUR, etc., is required for an opportunity to appear on any report. You do not have to have a value amount unless the report requires values. Depending on the report you select, you may also need data in the following opportunity fields in order for them to appear on selected reports.
- A forecast close date.
- An opportunity value.
- The probability of winning.
Also, make sure that you've chosen the correct timeframe (This Month, Last Month, Last Quarter, Last Year, All Time) for the dates corresponding to the selected report. Some reports use the Forecast Close Date field, while others use the date the opportunity was created or the date it was marked closed by changing the status to Won or Lost.