Try not to use a list for formulas. First create a system module that uses that list. Create a line item that has the ITEM. Whenever you use LOOKUP or SUM, you have to use a list formatted line item. Create another Line Item that gets the PARENT of the ITEM. In this case you'll look up the PARENT in your system module.
You have a list as shown below. I can't tell what it is so we'll just say "tasks". Tasks have a Project Parent.
Create a system module for tasks that contains two line items:
Task = ITEM(Tasks)
Project = PARENT(Task)
In your planning module you can now look up any Project if you know the Task Number.