Hi Imran,
What is the logic you are using in [Var_Incomplete] and [Var Completions]?
Check "Avoid duplicate row aggregation option" in Table display properties in order to avoid #multivalue in this block temporarily and see why this formula is returning multiple values (it would show what values are trying to get displayed in once cell)
Also try to put the formula =DaysBetween([Assignment Due Date];CurrentDate()) in the same block in a separate column and see what is returned in it, just to check if this part is working fine.
Thanks
Mallik