If this is the first time that you've used the service on this particular phone, then make sure that the enrollment process has been completed and then try again.
If you've used the service on this phone before and cannot login, then make sure that phone is not locked. If it is unlocked, then you may need to restart the mobile device and try again.
Make sure that you're using the correct mobile device. If you're using a new device (even if you have the same phone number), then reactivate Duo Mobile for the new device. (If you're changing types of phone, such as going from an Android to an iPhone, then select the new type of phone before reactivating.)
If the service is still not working, then contact the IT Service Desk.