Hello @macalca , you are the second one in a short amount of time that has reported a WhatsApp problem on the FP6 
, see Unable to activate whatsApp on fairphone 6 with 3.6
I unfortunately do not have a solution. Similar problems have been reported before a couple of years ago, see You need the official WhatsApp to log in erroror message
@Manoj : This may be something the developers should look into - maybe WhatsApp currently recognizes the FP6 (and other devices) with /e/OS where WhatsApp is used for the first time as ācompromisedā or something similar?