Could not find a part of the path 'D:\OLMS\'

Protected Sub btnadd_sylbus_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnadd_sylbus.Click
    Dim extension As String = System.IO.Path.GetExtension(FileUpload_sylbus.PostedFile.FileName).ToLower()
    Dim Type As String = Nothing
    If (extension = ".pdf") Then
        Dim intFileNameLength As Integer
        Dim strFileNamePath As String
        Dim strFileNameOnly As String
        strFileNamePath = FileUpload_sylbus.PostedFile.FileName
        intFileNameLength = InStr(1, StrReverse(strFileNamePath), "\")
        strFileNameOnly = Mid(strFileNamePath, (Len(strFileNamePath) - intFileNameLength) + 2)
        Dim br As New BinaryReader(FileUpload_sylbus.PostedFile.InputStream)
        FileUpload_sylbus.PostedFile.SaveAs("D:\OLMS\" & strFileNameOnly)
        cmd.CommandText = "INSERT into Syllabus(sylbus_id, sylbus_name, file_name, content) values(@id,@name,@file,@cont)"
        cmd.Connection = cn
        cmd.Parameters.Add("@id", txtsylbus_id.Text)
        cmd.Parameters.Add("@name", txtsylbus_name.Text)
        cmd.Parameters.Add("@file", FileUpload_sylbus.FileName)
        cmd.Parameters.Add("@cont", br.ReadBytes(FileUpload_sylbus.PostedFile.ContentLength))
        cmd.ExecuteNonQuery()
        cn.Close()
        lbladd_sylbus.Visible = True
        lbladd_sylbus.Text = "File Upload Success."
        txtsylbus_id.Text = Nothing
        txtsylbus_name.Text = Nothing
    Else
        lbladd_sylbus.Visible = True
        lbladd_sylbus.Text = "Not a Valid file format"
    End If
End Sub

You could try this code

strFileNamePath = FileUpload_sylbus.PostedFile.FileName
strFileNameOnly = Path.GetFilename(strFileNamePath)
Dim newFileNamePath as String = Path.Combine("D:\OLMS", strFileNameOnly)
Dim br As New BinaryReader(FileUpload_sylbus.PostedFile.InputStream)
FileUpload_sylbus.PostedFile.SaveAs(newFileNamePath)

Now, if you still get errors in accessing the folder D:\OLMS then you have this path outside the root folder of your site and your machine configuration (search about MEDIUM TRUST) disallows writing outside the root folder. (Of course I am assuming that the folder D:\OLMS exists and the user under which the IIS ASP.NET service is running has all the permissions to write in that folder)