If you want to remind a Client that there is an unpaid Credit Request on their account, go to Accounting > Raised Credit Requests, open the Credit Request and click Send reminder. This opens the Send Credit Request Reminder panel, from where you can opt to send just a reminder, or resend the full Credit Request.

Sending a Credit Request Reminder.

Reminders can be sent by email automatically once an unpaid Credit Request has been outstanding for a set number of days. Go to System > Settings > Accounting > General and tick Automatic Payment Request Reminders. The wait is set by the Automatic Payment Request Reminders Delay (days) field below it, which defaults to 7 days.

Setting up an automatic Invoice reminder.

Note: this setting covers both Invoices and Credit Requests, which is why it is worded as payment requests rather than either one. The delay is also used when an automatic charge fails.

Note: an individual Client will only receive these reminders if Credit Request reminders are switched on in their own notification settings. If a Client isn't getting them while others are, check there first — see here for how to manage what your Clients are notified about.