why port 6699 is listening ?

To use variable with string map, it expects list argument. Also no curly braces for defining variables. Try below code

when HTTP_REQUEST {
log local0. "Request URL: [HTTP::host][HTTP::uri]"
if { [class match [HTTP::uri] starts_with intranet_redirect_datagroup] } {
set full_uri [HTTP::uri]
log local0. "full URI - $full_uri"
set old_uri [class match -name [HTTP::uri] starts_with intranet_redirect_datagroup]
set new_uri [class match -value [HTTP::uri] starts_with intranet_redirect_datagroup]
log local0. "old_uri - $old_uri"
log local0. "new_uri - $new_uri"
set rewrite_uri [string map [list "$old_uri" "$new_uri"] [HTTP::uri]]
log local0. "New full URI - $rewrite_uri" 
HTTP::respond 302 Location "https://cloud1.sharepoint.com$rewrite_uri" "
return
  }
}