For one reason or another, one can find oneself being unable to disable ISSU, for example, this can happen if ISSU is unfinished, and the firewall is set to standalone mode.
ISSU will be unable to be disabled because the device is not on chassis cluster, and setting the firewall to chassis cluster mode is not possible because ISSU is enabled,
ISSU does not disable when "request chassis cluster in-service-upgrade abort" command is run.
ISSU is a kernel process, its objects exist in kernel of JunosOS (FreeBSD).
You can check the values of these objects can be checked with "sysctl -a | grep issue" in Shell mode.
root@SRX% sysctl -a | grep issu
busy_timeout = 0 cmd_issue_err = 0 dma_load_err = 0 dma_start_err = 0
hw.re.issu_switchover_ready: 0
hw.re.issu_state: 16 <<<<<<<<<this is non-zero if device is not out of ISSU window.
hw.re.issu_error: 0
hw.re.issu_reconnect: 0
To change the value of "issu_state" run "sysctl -w hw.re.issu_state=0"
This will disable ISSU, but if the device is rebooted, ISSU will re-enable, so for example in the standalone scenario, ISSU can be disabled through kernel in order to permit the firewall to be set as chassis cluster, once rebooted and in chassis cluster mode ISSU can be disabled running the "request chassis cluster in-service-upgrade abort"