There are a lot of ways to do this --> if you always want it to go exactly 24 hours after the approval is created, every time (not "at least" 24 hours, but rather exactly at 24 hours), you'll need to add this as a workflow step with a timer...
...but I would say that creates a lot of maintenance for your flows and isn't the best solution, when the desire is a simple reminder for those users, who have outstanding approvals.
Instead, I would recommend a daily script that checks the sysapproval_approver table for any record where "created >= 24hrs ago" && "state == 'requested'", then trigger an event (you'll need to register the custom event) that triggers the reminder email.
Here are some links to help get you started:
Create a new event and register in event registry = Event Registry | ServiceNow Developers
Write a scheduled script to run daily = Automatically run a script of your choosing (servicenow.com)
Include the generation of the event you created, in the script = Generating Events | ServiceNow Developers
Create the email notification which is sent, when the event is fired = How To Create Email Notifications In ServiceNow - YouTube
