If you have only one Orbit in Bluetooth range this means that the Orbit you are trying to link to your account has already being linked to another account. An Orbit can only be linked to one account. In this case, you will need to remove the Orbit from the other account.
1) Login into the app using the other account credentials and remove the Orbit by pressing the 3 little vertical dot next to the Orbit. You will then get an option to 'Remove'
2) Login out and then login with the new credentials and add the Orbit again.
If you have multiple Orbit in Bluetooth range, it could be that the phone is picking up the Bluetooth from another Orbit than the one you are trying to add. The best option, is to move away from all the other Orbits and make sure that only the Orbit you are trying to add is in Bluetooth range. If the same message comes up again, it means that the Orbit is already linked to another account and will need to be removed from the old account