The task are you talking about is it a Catalog task created as part of that RITM ?  or Are you considering a RITM as a (work) task ?

If you have already a separate flow for your Catalog Item that you are submitting (Building B Level..) then in that flow itself you can add a wait condition to check if your catalog task or RITM is closed complete or not. Once, it's Closed , you can use Send Email action and design your email in the Flow designer itself.

If you don't want to edit the flow , you can still create a standalone notification to send it to the Requestor's manager once the RITM is Closed.

If my response helped you, kindly mark it as helpful and accept the solution.

Regards,

Nayan