Currently, you need to log into Plann using your Instagram Username and Password.
Because the login screen is directly from Instagram, it is completely controlled by them and shows the ability to login with your Facebook account.
We've spotted an issue that when you try to login using your Facebook account, nothing happens. We are working on getting this sorted for you.
In the meantime, you can definitely log into Plann by using your Instagram details!
Again, we are still investigating and will let you know when we have an update :)