Workfront Reporting: Days Until Due

can you share what you've been playing with? There are a number of different ways to achieve what you're looking for, as far as I know.

You should have picked one of the formulas from the calculated data expressions help article:

https://experienceleague.adobe.com/en/docs/workfront/using/reporting/reports/calculated-custom-data/calculated-data-expressions

You could have gone with DATEDIFF, WEEKDAYDIFF, or WORKMINUTESDIFF, for example.

You should have picked one of the wildcard variables from the wildcards help article:

https://experienceleague.adobe.com/en/docs/workfront/using/reporting/reports/report-elements/understand-wildcard-filter-variables

(either $$TODAY or $$NOW)

You should know to express your field as camelCase and in curly braces.

Assuming a standard scenario, this might boil down to something like WEEKDAYDIFF($$TODAY,{plannedCompletionDate}) -- but like I said... there are lots of other options that really depend on what you're looking for.