Android Question No such file or directory

Dim path As String = File.DirRootExternal & "MathTest"
   
   
    If File.Exists(path,"ListResults.txt") Then


    Else

        For i = 0 To 99
            listResult(i)="0"
        Next

        listResult(0)=dayTodayString & "|" & monthTodayString & "|" & yearTodayString & "|" & minutesInt & "|" & secondsInt & "|" & errorsInt

        'creo la cartella
        File.MakeDir(File.DirRootExternal,"MathTest")


        'salvo l'array nel file di testo
        File.WriteList(path,"ListResults.txt", listResult)

    Dim Folder As String = "MathTest"
    File.MakeDir(File.DirRootExternal, Folder)
    Dim FullPath As String = File.Combine(File.DirRootExternal, Folder)

    Dim lstResults As List
    lstResults.Initialize
    lstResults.Add(1)
    lstResults.Add(2)
    lstResults.Add(3)

    Dim FileName As String = "Results.txt"
    File.WriteList(FullPath, FileName, lstResults)

Dim path As String = File.DirRootExternal & "/MathTest"

Sub Globals
    Private rp As RuntimePermissions
End Sub

Sub Activity_Create(FirstTime As Boolean)   
    Dim Folder As String ="MathTest"
    File.MakeDir(File.DirRootExternal,Folder)
    Dim FullPath As String=File.Combine(File.DirRootExternal,Folder)
   
    rp.CheckAndRequest(rp.PERMISSION_WRITE_EXTERNAL_STORAGE)  '
    Wait For Activity_PermissionResult (Permission As String, Result As Boolean)
    If Result = False Then
        MsgboxAsync("No permission to access external storage", "")
        Return
    Else
        Dim lstResults As List
        lstResults.Initialize
        lstResults.Add(1)
        lstResults.Add(2)
        lstResults.Add(3)
        Dim FileName As String= "Results.txt"
        File.WriteList(FullPath,FileName,lstResults)
    End If
End Sub