You must have "Full Access" or "Limited Access - Financials" permissions.
Invoices
From the left navigation menu, click Financials > Invoicing.
Click Filter by Status to include Open, Past Due, or both statuses.
Select the invoices and click Send Reminder.
Enter a message and click Send Reminder.
Orders
From the left navigation menu, click Financials > Orders.
Use the Search bar or Filters to find the needed order.
Click the Sale number.
In the upper-right corner, click Send Reminder.
Enter a message and click Send Reminder.