There are several possibilities you couldn’t find debtor when creating invoice.
1. The debtor is inactive
Normally when a debtor is set to “Inactive” for the reason of not allowing further sales to such debtor. To activate it, you may check the checkbox of “Active” in Debtor Maintenance.
2. The debtor code is group company
Go to Debtor Maintenance, observe the checkbox of “Group Company” When an account is defined as Group Company it serves the purpose of Control Account, thus not allowed to be invoiced.
3. The debtor is created under normal account
To select a debtor code under sales document, you need to create it in Debtor Maintenance (AR Menu). If a debtor account is created using Account Maintenance (GL Menu), it will not appear for selection in sales document.
4. The debtor has been filtered out according to the setting of Filtered by Salesman
You might have set a particular allowed to access to his own customers only. That’s why debtors not belonged to this user will not be showed. This feature is available with the module of Filtered by Salesman. The setting is at Tools > Options > apply filter by agent in lookup editor.