Hi Alex,
Can you confirm that your user has been provisioned a Navigator license?
If so, can you give me some more details about your device? - are you using Android or iOS?
If you already have a Navigator license provisioned, and you are using Android, it's possible you are running into a known issue that we have when the Organization Setting to "Allow anonymous access to your organization's website" is unchecked. This issue is specific to Android, and we are actively working on a fix. To test if you are running into this, you could enable anonymous access to your org, and try to sign in again.
Thanks,
Heather