See this article on how to fix unable to add account
If you are unable to pass the Instagram checkpoint starting from 7th of December 2017, you should know that it is not an error on your end but a suspected bug on Instagram.
We’ve received this report from several customers. And it seems to be happening to some accounts only.
It’s not related to our service, other services like ours also have this problem right now. Apparently, Instagram has made some changes on its end which we are unable to inspect right now.
The issue is observed in some official Instagram applications too. So it can be said it’s a bug on Instagram right now.
We are researching the issue with the whole team and will try to find a solution as soon as possible.
Thanks for your understanding.