[A] If you are facing the issue that document number is blank while creating the document as shown below that means someone from your team has changed the document number to be manual.

[B] Please check this with your team if anyone has done these changes if you wish to change it back to automatic you can change the same from the settings module.

[C] If you do not have the permission of settings module or to edit the settings then please contact your senior person who has all the permissions.

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