This article helps with latest OS recommendation for PAA installation.
For the latest PAA release - supported OS -> https://www.juniper.net/documentation/us/en/software/active-assurance4.4/paa-install/topics/concept/install-os-software.html
Latest Document :: https://www.juniper.net/documentation/us/en/software/routing-active-testing4.6/rat-install/topics/concept/install-os-software.html >> Says the same.
Please contact JTAC support for any queries or concerns.