10001385 
Message
Enter valid number; document number already exists
Diagnosis
In a sales or purchasing document, in the No. dropdown list, you have chosen Manual.
In the field to the right of the No. dropdown list, you have entered a document number.
However, the number you entered already exists in SAP Business One. You cannot enter a document number that has already been used for the same document type in SAP Business One.
Procedure
In the field to the right of the No. dropdown list, enter a valid document number; in other words, enter a number that does not already exist for the specific document type in SAP Business One.
Note
To set manual numbering for China, Japan, Korea, Singapore, India, or Brazil, you must have full authorization to document manual numbering. If so, in the No. dropdown list, select Manual.
To check which document numbers already exist in SAP Business One, do one of the following:
Check the number of the last added document, as follows:
From the SAP Business One Main Menu, open a new document of the same type as the document whose number you want to enter manually.
In the toolbar, use
(Previous Record) and 
(Next Record)
to scroll through the existing documents. In the No. field in the upper right corner of the documents, you can see which document numbers have been used.
Check the Document Numbering - Setup window.
NoteIf you are not a superuser in SAP Business One, you may need to ask a superuser to perform this procedure.
End of the note.From the SAP Business One Main Menu, choose
Administration 
System Initialization Document Numbering 
.In the Document Numbering - Setup window, in the Document column, find the row of the document whose existing numbers you want to check.
In the First No. column, you can see the first existing document number. In the Next No. column, you can see the number that will be assigned to the next document.
The existing document numbers include a range from the first number to the next number, minus one. You cannot add these numbers manually to a new document.