You are so close!
You are going to need to run a loop. A loop essentially runs a specific action for each item in a list.
For example, if we looped over this list 1, 2, 3, 4 and multiplied each value by 4, we would get:
4,8,12,16
Whereas, if you just tried to multiply that whole list by 4, you would get an error. You can’t multiply a LIST by numbers. You can only multiply numbers by numbers
You were trying to do the equivalent. When you run a ModifyRows() action, you are returned a LIST of values.
As you have your ModifyRows() set up, it’s going to try to update them all at once to the SAME value. And when you said TaskView.DueDate + 7 you were trying to ADD the number 7 to a giant list of values.
How we loop in Coda is with a formula called forEach() - Heres some further documentation on how that formula works! 