This is straight forward, go to v18 and then onto v21. I have done this very thing myself many times. You cannot jump straight to v21. A max. of a 3 version hop is permitted, but no more.
Sent: 11-14-2022 23:12
From: Unknown User
Subject: Juniper SRX340 Upgrade Path
I am trying to upgrade the Junos OS of my SRX340 firewall from 15.1X49-D60.7 to 21.2R3-S2.
Since the two versions are far apart, I'm worried that there might be some problem.
Can I upgrade directly or which kind of upgrade path should I follow?