Hi,
SSLVPN on the SRX is only supported for NCP client.
my point was that NCP client support started from 15.1X49-D80 onwards.
So if you wish to use SSL VPN termination on the SRX then you will have to use NCP client and the junos version should be 15.1X49-D80 atleast.
hope this clears any confusion.
regards,
Guru Prasad