Solution:

To log in and host meetings from the mobile app, you will need to use an account with host privileges. Normally, you may still join meetings using the mobile meetings app without logging in.

If you receive this error when just trying to join the meeting, and you are not trying to log in, please check with the meeting host. The Webex site may have the setting 'Require users to have an account when joining by phone' enabled to require you to log in before attending the meetings.
To join the meeting, request you host to set up an 'Attendee only' account for you and login to the app to the meeting.