First you go to viber support https://help.viber.com/hc/en-us/requests/new?ticket_form_id=5377663333917 and submit a request, you can put anything you want, i.e for description just put “viber activation failed”.
Then you will get an email from viber telling you that you will recieve a 6 digit code for the activation.
What you need to do is reply to that message, my message was “There is no input available for the code, only an error “activation failed” is visible”.
After that you will get a message to re-install viber and restart your device, which you will do.
After re-installing and restarting go to viber and enter your number and you will get the input where you can enter the code, but the problem is that you don’t get the code, the fix to that is easy, you just need to contact viber support again, telling them that your viber activation failed.
After thay, they will send you an email telling you that you will get a 6 digit code which you will get.
And all you need to do is enter that 6 digit code and done, viber should be working.
Hope I helped someone.