So we just got our son a vivofit jnr 2 for his birthday, tried setting up on my wife's phone (iphone11) same error as you describe.. she also has issues with her own Garmin watch.
Then I downloaded the app on my phone (iphoneX) it connected straight away and I was able to complete the connection.
we have exactly the same issue. 2 out of 3 vivo fit jnr devices work seamlessly. 1 of them says it's linked to another account. Option is to reset it but upon selecting reset a message appears saying to 'verify your connection and try again'. We've tried removing the battery, pressing and holding the button etc etc. Deleting the app and re installing, deleting bluetooth pairing and re-pairing. Nothing!!