Please be more specific. See Rule #2.

Show us the code that is running when error comes up otherwise we have no frame of reference

Have you tried wrapping the ID with the NZ Function?

hi again
0 Value is not an option there get to be a Value
I'm using 3 synchronized subForms
I'm using Father Son connection
this is the Code I use

Private Sub Form_BeforeInsert(Cancel As Integer)
Dim HighestSectionNumber As Double

If Not IsNull(SectionNumber) Then
   Exit Sub
End If
    
    HighestSectionNumber = Nz(DMax("SectionNumber", "ChecklistT", "SectionID=" & SectionID), 0)
    SectionNumber= HighestSectionNumber + 1

End Sub
I get SectionID=Null instead of a Value

Change

    If Not IsNull(SectionNumber) Then

To

    If IsNull(SectionNumber) Then

At the moment you are exiting the Sub if SectionNumber has a value.

Which lesson of Fitness 76 is this from?
What is the timestamp?

the form name is RoutineDetailF
(its a subForm  within ProgramRoutineF)
timestamp is 7:37

hi All
I think I found the solution to my problem.
My Forms are based on queries as data source.
I changed that into tables and Its worked problem is solved.
thank you All

Glad you got it sorted out. Kevin was right to ask which lesson you were following, because that code assumes the parent ID has already been saved and is available when the child record is inserted.

Using a query as a form's Record Source is not automatically a problem, but it can complicate things if the query is not updateable or does not expose the fields Access needs to maintain the parent/child relationship. In this case, basing the forms directly on the tables was the cleanest fix.