Where & when do you trigger the microflow?
You could debug the microflow as suggested by Mike
See: https://docs.mendix.com/howto/monitoring-troubleshooting/debug-microflows
//Update
Instead of using the expression
$Orderline/Module1.OrderLine_ActiveStatusPricing/Module1.ActiveStatusPricing/Price_Per_Mil
Retrieve the object ActiveStatusPricing by association first and use the expression
$Orderline/Total_Unit_Count div 1000 * $ActiveStatusPricing/Price_Per_Mil
run app, and debug it. See if you retrieve an object ActiveStatusPricing and if found what the value of attribute Price_Per_Mil is.
My assumption is that either no object is retrieved or the attribute value is empty.