Syntax error in string manipulation

[angelica]

Hey,

Some help please!

I am trying an encrypting exercise which takes the txtmessage and encrypts it in characterarray.
IStartAt is the starting point and by taking one character at a time from txtmessage I need to insert it in characterarr\y allowing a gap in between which is IUseGap. KeyGap and Keyfirst will be the next two characters after txtmessage and when insert have no gap in between.

I think the logic I done is good however, I get syntax errors on the red lines.

Please Help me get this right. Many thanks

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnsecrettMessage.Click
        'Splitletters()
        Dim KeyFirst, KeyGap, secretmessage As String
        Dim encodestring As String = txtmessage.Text
        lStartAt = 200 + lUse5th
        Dim lPos As Long
        KeyFirst = Asc(encodestring(0))
        KeyGap = (Asc(lUseGap))
        encodestring += String.Format(KeyFirst + KeyGap)
        For lPos = 1 To encodestring.Length
            characterarray.Substring(((lPos - 1) * lUseGap) + lStartAt) = encodestring.Substring(lPos - 1)     

       If (encodestring.Length - 2 = True) Then
characterarray.replace (1Pos+1,KeyFirst)  

            End If

            If (encodestring.Length - 1 = True) Then
characterarray.replace (1Pos+2,KeyGap)      
        End If
        Next

        secretmessage = characterarray.ToString

    End Sub