When you create a customer for the first time with the Customer Add app, you may be expecting to use the email address or phone number you just added to be available so that you can send the receipt by email or text message.
In this situation, the Register or Sale app has not synced with Clover yet and pulled in the customer information yet. If you tap the "Sync" button or wait a few second, the customer information is typically synced and available for use.