WAN interface not available in "Incoming Interface" list when use L2TP/IPsec Template

Hi,

I am running 6.4.4. When I try to create a remote access IPsec tunnel using either Native Android or Windows template. None of the active interface/Zone is in the list of the "Incoming Interface" drop down list, only disabled interfaces are listed. All other templates are ok, just those 2 for some reason. I noticed those 2 are L2TP tunnel. Does any one know why? Thanks.