Two-step verification. If you have such a login feature, you will be asked to enter a security code (send via email or text message (SMS) - depends on your provider). Enter this code in the recently added loyalty program. If you don't see a place to enter the code, try "pull to refresh" the screen. This would update it, and you should see a place to enter the code.
Try logging in with the credentials you use on the airline website. If you are able to do it there but not within the app, please send feedback from the app's settings to give our Support Team a heads-up.
If you see an Unknown error (or any other error type) try to refresh the page ("pull to refresh" the screen with the list of the connected programs), and then check it again for the new status. Or, try to delete and add it again. See here how you can do it.