You can not change the FieldName parameter of Bug_FieldChange. It should look like this:
Sub Bug_FieldChange(FieldName)
On Error Resume Next
If Bug_Fields("BG_USER_01").Value = "Task" Then
Bug_Fields("BG_USER_O4").IsRequired = False
Else
Bug_Fields("BG_USER_O4").IsRequired = True
End If
On Error GoTo 0
End Sub
Also, you must have the code in Bug_MoveTo as well.
Sub Bug_FieldChange(FieldName)
On Error Resume Next
If Bug_Fields("BG_USER_01").Value = "Task" Then
Bug_Fields("BG_USER_O4").IsRequired = False
Else
Bug_Fields("BG_USER_O4").IsRequired = True
End If
On Error GoTo 0
End Sub
Also, you must have the code in Bug_MoveTo as well.