For your optional parameters state the value to return if the parameter is omitted.
For example:

Optional rsServiceNote As String = ""

Hi Kevin

I did what you said :

Details

Private Sub PrintRoster( _
    rsDayOfWeekID As Integer, _
    rsAllBlocks As Boolean, _
    rsSpanish As Boolean, _
    rsReligionCode As String, _
    rsUseservice As Boolean, _
    rsServiceName As String, _
    rsServiceMessage As String, _
    rsTimeSlot As Date, _
    rsLocation As String, _
    Optional rsServiceNote As String = "", _
    Optional rsSelectedBlocks As String = "", _
    Optional rsAreYouSure As Boolean = False)

But I am still getting the same error.
I tried to deal with it before calling the sub but it does not seem to set the value:

Private Sub SelectBtn_Click()
    If ServiceNote = Null Then
        ServiceNote = " "
    End If
    MsgBox "-" & ServiceNote & "-"
    
    If IsNull(SelectedBlocks) Then
        SelectedBlocks = " "
    End If
    PrintRoster ..........


rs usually means recordset, what is it denoting here?

Hi Alex

I call "PrintRoster" from 2 places, see code below, one of them is a recordset Loop to print all the reports.
You are correct the "rs" is not appropriate in the parameters. I will re name as soon as I get this to work.

many thanks

Details

Public Sub PrintALLBtn_Click()

    Me.Requery
    If MsgBox("you are about to print ALL the selected rosters. Are you sure?", _
      vbYesNoCancel + vbExclamation, "Warning") <> vbYes Then
    Exit Sub
    End If
    
    Dim rs As Recordset
    Set rs = CurrentDb.OpenRecordset("SELECT * FROM ServiceQ WHERE " & Filter)
    While Not rs.EOF
        
        If rs!PrintAll Then
'MsgBox "Print = " & rs!PrintAll & " -- " & rs!ServiceMessage
            PrintRoster rs!DayOfWeekID, _
            rs!AllBlocks, _
            rs!Spanish, _
            rs!ReligionCode, _
            rs!Useservice, _
            rs!ServiceName, _
            rs!ServiceMessage, _
            rs!TimeSlot, _
            rs!Location, _
            rs!ServiceNote, _
            rs!SelectedBlocks, _
            False
            rs.MoveNext
        Else
            rs.MoveNext
        End If
    Wend
    rs.Close
    Set rs = Nothing

End Sub

Private Sub SelectBtn_Click()
    If ServiceNote = Null Then
        ServiceNote = " "
    End If
    MsgBox "-" & ServiceNote & "-"
    
    If IsNull(SelectedBlocks) Then
        SelectedBlocks = " "
    End If
    PrintRoster _
        DayOfWeekID, _
        AllBlocks, _
        Spanish, _
        ReligionCode, _
        Useservice, _
        ServiceName, _
        ServiceMessage, _
        TimeSlot, _
        Location, _
        ServiceNote, _
        SelectedBlocks, _
        True
    
End Sub

A string variable cannot be null, and that's why you get the "cannot be null" error.  To declare an optional parameter that can be null and defaults to null, use the Variant data type, and write:

     Optional v As Variant = Null

Thank you Kevin, that worked.

Many thanks

Tell Richard to add you to the tip jar.

Thanks.  My efforts here are completely voluntary, so no need to tip.  I'm retired, and used Access in my old job; and I just want to give back to the Access community in whatever minor ways.  I post only here on this site.

Other moderators actually know all of Richard"s courses and post a lot more often than I do.  So they deserve the tips more than I do.

Thanks again Kevin
You have helped me a lot in the past.

I have been away from writing code for over 30 years. I just started learning VBA a few months ago. Richard courses is the reason I can write these applications for my work.

you guys came to my rescue many times.