À priori, la date est sous forme de texte, il faut la transformer en date.
For i = 1 To Application.CountA(Range("A:A"))
If Cells(i + 1, 5) = "PC" Or Cells(i + 1, 5) = "IMP" Then
Cells(i + 1, 15) = CDate(Cells(i + 1, 4)) + 14
ElseIf Cells(i + 1, 5) = "MESSAGERIE" Or Cells(i + 1, 5) = "NAVIGATION" Then
Cells(i + 1, 15) = CDate(Cells(i + 1, 4)) + 2
End If
Next i