@Mark Brogna 

you cannot create flow on 'sc_req_item' table

Did you create a subflow which accepts the table and the RITM sysId to trigger?

Why not use "Submit Catalog Item Request" flow action and it will generate a fresh REQ and RITM

Then you can attach your flow to that catalog item which you are using in earlier step.

If my response helped please mark it correct and close the thread so that it benefits future readers.