This is all about the Limit to List Property setting of the Combo Box. When this property value is set to Yes you cannot type a new value into the combo box control. The user is forced to select/type an item from the drop-down list of the combo box.

Typing a new value into the combo box control will trigger an error message and will force you to select an item from the existing list.

Names of Months or Weeks etc. in a combo box are of constant nature and never changes or an addition is not required. But, names of new employees or names of Products, clients and so on need to be added to the combo box list, as and when the need arises.

A new item can be added to the Source Table of the combo box directly through a separate Form.  The user has to open source table editing form and add a new record.  Even if the user is prepared to do that the new item added on the source table will not appear on the combo box list on the form immediately. The user must close and open the form with the combo box to refresh and make the new item appear in the list. It is not a user-friendly approach and lot of time is wasted.

But, we can make it very easy to the user with a VBA Program. The user can type a new value into the combo box itself and with his/her permission we can add the new item into the source table and refresh the combo box instantly.

When a new value is typed into the combo box the Limit to List property setting will run our program from On Not in List Event Procedure.  After getting confirmation from the user we can add the new item into the combo box source table and refresh the combo box.

The following program adds ProductID and Product Name into the Products Table and refreshes the cboProd Combo box to make the new item appear in the list:

Private Sub cboProd_NotInList(NewData As String, Response As Integer)
Dim strProd As String, strName as String
Dim rst As Recordset, db As Database
Dim msg as string

On Error Goto cboProd_NotInList_Err

'continue without displaying error message
Response = acDataErrContinue

strProd = NewData
msg = "Product ID: " & strProd & " Not found in List!" & vbCr & vbCr & "Add it in Source File...?"

If MsgBox(msg, vbDefaultButton2 + vbYesNo + vbQuestion, "cboProd_NotinList()") = vbYes Then
    'Get Product Name from User
    strName=""
    'Repeat the loop if user presses enter key without entering a value
    Do While strName=""
        strName = InputBox("Product Name: ","cboProd_NotinList()","")
    Loop
    'Add ProductID and Name to the source Table
    Set db = CurrentDb
    Set rst = db.OpenRecordset("Products", dbOpenDynaset)
    With rst

      .AddNew
      ![ProductID] = strProd
      ![PName] = strName
      .Update
      .Close
    End With
    
    'make combobox control source empty
    'in preparation to refresh the combobox
    Me!cboProd = Null
    'refresh the combobox
    Me.cboProd.Requery
    'now the combobox list have the new entry
    'place the new code in the combobox control source
    Me![cboProd] = strProd
 
    Set rst = Nothing
    Set db = Nothing
Else
   'if user refuse to add the new entry into source file
   'then display the error message and exit
   Response = acDataErrDisplay
End If

cboProd_NotInList_Exit:
Exit Sub

cboProd_NotInList_Err:
MsgBox Err & " : " & Err.Description,,"cboProd_NotInList()"
Resume cboProd_NotInList_Exit

End Sub

The above code is run from On Not in List Event Procedure of the cboProd Combo box.