probier mal damit:

Option Explicit

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
  
  Dim strRegNo As String
  Dim rngZelle As Excel.Range
  
  If Target.Address = "$D$14" Then
    
    strRegNo = Worksheets("Registernummer").Range("B2").Value
    
    With Worksheets("Data")
      
      On Error Resume Next
      Set rngZelle = .Columns("C").Find( _
                        What:=strRegNo, _
                        LookIn:=xlValues, _
                        LookAt:=xlWhole, _
                        SearchOrder:=xlByColumns)
      On Error GoTo 0
      
      If rngZelle Is Nothing Then
        Cancel = True
        Call MsgBox("Kein Treffer für '" & strRegNo & "'.", vbExclamation)
        Exit Sub
      End If
      
      .Activate
      .Cells(rngZelle.Row, "AR").Activate
      
    End With
    
  End If

End Sub