Hi @Zach N I appreciate the suggestion! After further review, I think my issue is that my Lookup returns 0 records. Please see my flow below:

I am first doing a Look Up in the cmn_location table to find the correct location. After finding the correct location, I'm doing a For Each to grab every single Serial Number in that location. Then I do another Look Up in our requests table to see if a Serial Number has been reported on a request for license renewal. If it doesn't have any requests submitted within the last 30 days, then set flow variables. This is where my script starts, to assign a flow variable a list of Serial Numbers that have not been reported on recently. 

I don't know if I need to do the Lookup from step 8, or step 10 within the script. I think my challenge is that the Lookup from step 10 returns 0 records, so I need to be able to somehow get the script to do a Lookup in step 8, compare if there is a matching record in the Lookup in step 10, and if not, add the Serial Number to the variable.