I did of cpurse but not able to download due to the below error:
You have encountered this error because your account privileges do not permit access to the information or service requested. Software download entitlement is granted for customers under the following scenarios.
Juniper Product within the first 90 days of the hardware warranty period. Juniper Product which is currently under an active maintenance contract. Juniper Standalone Software Subscription which is currently active.
Basically what you seing is the phone-home client for zero touch provisioning. To disable this, you have to login via serial console, go into configuration mode and delete the 'system phone-home' part of the configuration.
I expect your swith is with the factory default meaning something like the following commands (login with root and no password):
then you should be able to access the basic j-web package on your EX3400. Regarding software access, create a case for customer care mentioning the serial number for your device... you will then be given access to download of software for your EX switch.