Dynamic-vpn is not supported on SRX1500 - only on branch SRX platforms (SRX100-SRX650).
There will be an option for remote access VPN via the 'NCP client' in a few weeks with the release on 15.1X49-D80 - but the license scheme has changed as well, so your SRX-RAC-LTU licenses are not usable for this.
If your reseller has sold you dynamic VPN licenses for your SRX1500 I would ask them to credit these as they are unusable for this platform.
-- Best regards,
Jonas Hauge Jensen Systems Engineer, SEC DATACOM A/S (Denmark)