Option Explicit
Private Sub CommandButton1_Click()
Dim X As Range, C As Range
Set C = Sheets("Tableau").Columns(2).Find(What:=[C8])
Set X = [C6]
If Not C Is Nothing And [C12] = True Then
C.Offset(, X) = C.Offset(, X) + [C10].Value
Else
C.Offset(, X) = [C10].Value
End If
End Sub