Hmm ... I don't know why it doesn't want to work. However, I've uploaded an alternative firmware file which sets the "Reset/Downgrade" flag. Maybe this one works. Otherwise, try to downgrade to a previous D2 Delta version using the Backdate-firmwares (they work the same - rename to GUPDATE.GCD and put into GARMIN folder on watch) and then try to convert from that older version to fenix 5 Plus.