Which firmware you HAD before?
All my Sonoff POW have 2.02 or 2.04 firmware version, and on the SonOTA is written do not try it if firmware above 1.6 … ?
I don’t know how to get my pow2 version but I ended up with this https://i.imgur.com/5fiHNNh.png 
Any help?
you may have to flash it wired… I think sonota is discontinued
It seems that flashing over the air is not possible for all firmware versions.
How to tell what firmware versions I have on my Sonoff Basics (received yesterday)
You can check in EWeLink app, under settings menu for each device