You use the word "selects"
Is user selecting from a menu of choices, like a Data Validation Dropdown
list or is user simply typing "yes" in a cell?
Either way, to get an actual pop up message you would need event code to
remind user.
This is a reminder only.
Private Sub Worksheet_Change(ByVal Target As Range)
Const myRange As String = "A1" ' "A1:A10" if a range
On Error GoTo endit
Application.EnableEvents = False
If Not Intersect(Target, Me.Range(myRange)) Is Nothing Then
If Target.Value = "yes" Then
MsgBox "You must fill in " & Target.Offset(0, 1).Address
End If
End If
endit:
Application.EnableEvents = True
End Sub
Right-click on the sheet tab and "View Code". Copy/paste the above into
that sheet module.
Edit to suit. Alt + q to return to Excel.
Gord Dibben MS Excel MVP