Discover why FIPS mode may fail to function in this detailed knowledge base article, exploring a single scenario and its underlying causes.
When hen dealing with FIPS take into consideration that not all versions are FIPS compliant. Versions that are at the moment FIPS compliant.
Ensure that you are using a certified FIPS release version of the software, as FIPS mode won't be enabled on uncertified versions.
https://apps.juniper.net/compliance/fips.html?p=SRX340&t=product
Refer to this link above, click on filter, right hand side filter option you will see the SRX model, select it and then you will see which versions are available for each SRX
Refer to the following documentation for additional details on this:
Compliance Advisor
Cryptographic Module Validation Program CMVP
FIPS Evaluated Configuration Guide for SRX345, SRX345 Dual AC, SRX380, and SRX1500 Devices