Hi,
Policy based VPNs are supported on the 300 series devices starting from 15.1X49-D50.
This can be found under New Features -> VPNs
I would suggest that you upgrade the SRX to 15.1X49-D70 which is a much more stable release to use policy based VPN.
The D70 image can be downloaded form the following location :-
https://www.juniper.net/support/downloads/?p=srx320#sw
Hope this helps !
Regards,
Sahil Sharma
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