This means that your Sales Forecast was too far from Actual Demand to return a positive value figure.
For example:
IF Forecast Sales last year was 70,500 units
AND Product Demand was 26,284 units
The difference is 70,500 – 26,284 = 44,216
As the difference is bigger than total actual demand we return zero for Demand Forecast Accuracy otherwise the calculation would return a negative value.