JunOS 15 has a different way of booting into single user mode. You need to get into u-boot, delete a keyword then perform the "standard" procedure.
After the device shows USB devices, press Enter or any key (I don't remember which) to get u-boot's => prompt.
Then
printenv
There should be an entry for
boot_unattended=0
If there is
setenv boot_unattended
boot
Now when you see "Hit [Enter] to boot immediately, or space bar for command prompt." press space bar to get the loader> prompt. Here you can issue
boot -s
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KEN LUI
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