assistance in VBA that will enable me to have mandatory fields and lock rows when complete

nsb_500 

Please replace the existing code with the following code and let me know if that takes care of both your requirements.

The password is set on the first line as a constant string variable i.e. the password right now is nsb_500, if you want to change the password, first manually unprotect the Details Sheet with the password nsb_500 and then change the password on the first line of the code. Run the macro once and it will set the new password.

Const PW As String = "nsb_500"

Private Sub CommandButton1_Click()
Dim wbSource        As Workbook
Dim wbDest          As Workbook
Dim wsSource        As Worksheet
Dim wsDest          As Worksheet
Dim name As String
Dim invoiceno As Long
Dim paymentmethod As String
Dim collectedby As String
Dim item As String
Dim item2 As String
Dim r As Long
Dim mydate As String
Dim path As String
Dim myfilename As String

Application.ScreenUpdating = False

Set wbSource = ThisWorkbook
Set wsSource = wbSource.Worksheets("Details")

lastrow = wsSource.Range("A" & Rows.Count).End(xlUp).Row

'Unprotecting the Details Sheet
wsSource.Unprotect Password:=PW

For r = 2 To lastrow
    If wsSource.Cells(r, 13).Value = "done" Then GoTo Nextrow
    
    If Application.CountBlank(wsSource.Range("A" & r & ":B" & r)) > 0 Then
        wsSource.Cells(r, 13).Value = "All the fileds are mandatory!"
        wsSource.Range("A" & r & ":M" & r).Locked = False
    ElseIf Application.CountBlank(Range("D" & r & ":K" & r)) > 0 Then
        wsSource.Cells(r, 13).Value = "All the fileds are mandatory!"
        wsSource.Range("A" & r & ":M" & r).Locked = False
    Else
        name = wsSource.Cells(r, 1).Value
        invoiceno = wsSource.Cells(r, 2).Value
        paymentmethod = wsSource.Cells(r, 4).Value
        collectedby = wsSource.Cells(r, 5).Value
        quantity = wsSource.Cells(r, 6).Value
        quantity2 = wsSource.Cells(r, 7).Value
        item = wsSource.Cells(r, 8).Value
        item2 = wsSource.Cells(r, 9).Value
        UnitPrice = wsSource.Cells(r, 10).Value
        UnitPrice2 = wsSource.Cells(r, 11).Value
        
        Application.DisplayAlerts = False
        
        Set wbDest = Workbooks.Open("C:\Users\Hp\Documents\receipt\receipt.xlsx")
        Set wsDest = wbDest.Worksheets("test")
        
        With wsDest
            .Range("A9").Value = quantity
            .Range("A10").Value = quantity2
            .Range("B3").Value = name
            .Range("B6").Value = paymentmethod
            .Range("B9").Value = item
            .Range("B10").Value = item2
            .Range("C9").Value = UnitPrice
            .Range("C10").Value = UnitPrice2
            .Range("D3").Value = invoiceno
            .Range("D6").Value = collectedby
        End With
        
        path = "C:\Users\Hp\Documents\receipt\"
        mydate = Date
        mydate = Format(mydate, "dd_mm_yyyy")
        
        wbDest.SaveAs Filename:=path & invoiceno & " - " & name & " - " & mydate & ".xlsx"
        myfilename = wbDest.FullName
        'SetAttr myfilename, vbReadOnly
    
        Application.DisplayAlerts = True
        wbDest.PrintOut copies:=1
        wbDest.Close SaveChanges:=False
        
        wsSource.Cells(r, 13).Value = "done"
        wsSource.Range("A" & r & ":M" & r).Locked = True
        
    End If
Nextrow:

Next r

'Protecting the Details Sheet
wsSource.Protect Password:=PW
End Sub