Please follow the steps included below for creating a rollup to count how many related tasks were open and had a due date in the future.
To achieve this requirement, the user needs to create one custom field.
Create a formula field on the Task/Activity Object, Formula = IF( IsClosed = FALSE && ActivityDate > TODAY(), 'TRUE', 'FALSE')
Open PWR Rollups application
Create a new roll-up and give the appropriate name and description
Select the source object as Task and the destination object as Opportunity, and click next
Select rollup type as Count and select the destination field where you want to show the result
Now, click on the 'Add Condition' button in the custom filter section
Please refer to the screenshot below for the filter specifications
You will need to add 2 filters to attain the requirement which can be done using the 'Add Condtion' button present at bottom
Check the checkbox to make rollup active then click on save
Now you have successfully created the rollup to fulfill your requirement