Failed to connect to ubus

Hi, I try to flash my device in recovery mode and get this error.
How can I work around/debug/fix this issue?

root@(none):/tmp# sysupgrade -n lede-ramips-mt7620-wt3020-4M-squashfs-sysupgrade.bin 
killall: watchdog: no process killed
Commencing upgrade. All shell sessions will be closed now.
Failed to connect to ubus

See these steps @slh provided in this thread...

Make sure you use lowercase y when answering the firstboot prompt.

It worked after executing firstboot. - thanks