Can the system remind me before a held flight booking expires so I do not lose the seat?

Yes, in most cases the system can flag a held booking before its deadline passes, as long as the supplier tells us when that deadline is.

When you hold a flight without paying, the airline or the consolidator usually returns a ticketing time limit along with the booking confirmation. If that date and time comes back, it can be saved against the booking and used to warn your staff, keep the booking sitting in a pending list, or send a reminder email a set period before it runs out. The same idea works for hotel bookings that carry a free cancellation deadline, so your team knows the last safe moment to amend or drop one without a penalty.

The honest limit is that this depends on the supplier. Some do not send a time limit at all, some send one and then shorten it without telling anyone, and many low cost airlines do not allow a hold in the first place, so payment has to be taken straight away and there is nothing to track. Where no deadline comes back, your staff still have to work from whatever the supplier shows in its own portal. Time zones matter too, so it is worth confirming that reminders appear in your office's local time rather than the supplier's.

You also get a choice about what happens when the clock runs out. Some agencies want only a reminder so that a person decides, others want the booking released automatically, and a few want ticketing to go ahead on its own when the money is already collected. Which of those you can use depends on your plan and on what the supplier permits, so the next step is to check with each of your main suppliers whether their booking confirmation carries a hold deadline, then tell us which of those endings you want.