XL 2019 message d'alerte si une cellule est vide

chemin = "C:\"

chemin = ThisWorkbook.Path & "\"

With Application.FileDialog(msoFileDialogFolderPicker)
    If .Show Then chemin = .SelectedItems(1) & "\" 'dossier choisi
End With

Private Sub Workbook_Open()
With Sheets("COMMANDE").[G13]
    .Parent.Visible = xlSheetVisible 'si la feuille est masquée
    Application.Goto .Cells
    .Value = .Value 'lance la macro Worksheet_Change
End With
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim c As Range
ScrollArea = "" 'libère la sélection
Set c = [G13]
If Not Intersect(Target, c) Is Nothing Then _
    If Not IsEmpty(c) Then [A18].Select Else c.Select: ScrollArea = c.Address: MsgBox c.Address(0, 0) & " doit être remplie...": Exit Sub
For Each c In [H18:H119] 'plage adaptable au besoin
    If IsEmpty(c) And (IsNumeric(CStr(c(1, 6))) Or CStr(c(1, 6)) = "REPRISE RMA TEK1.0") _
        Then c.Select: ScrollArea = c.Address: Exit Sub 'bloque la sélection
Next
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Intersect(Target, [G13]) Is Nothing Then Worksheet_Change ActiveCell 'lance la macro
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim c As Range
ScrollArea = "" 'libère la sélection
Set c = [G13]
If Not Intersect(Target, c) Is Nothing Then _
    If Not IsEmpty(c) Then Set Target = [H7] Else c.Select: ScrollArea = c.Address: MsgBox "MERCI DE SPECIFIER LE TYPE DE COMMANDE POUR CONTINUER": Exit Sub
Set c = [H7]
If Not Intersect(Target, c) Is Nothing Then _
    If IsDate(c) And c >= [H5] Then [A18].Select Else c.Select: ScrollArea = c.Address: MsgBox "LA DATE DE LIVRAISON DOIT ÊTRE AU PLUS TÔT " & UCase([H5].Text): Exit Sub
For Each c In [H18:H119] 'plage adaptable au besoin
    If IsEmpty(c) And (IsNumeric(CStr(c(1, 6))) Or CStr(c(1, 6)) = "05-REPRISE RMA TEK1.0") _
        Then c.Select: ScrollArea = c.Address: Exit Sub 'bloque la sélection
Next
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Intersect(Target, [G13,H7]) Is Nothing Then Worksheet_Change ActiveCell 'lance la macro
End Sub