Effort Has Not Been Launched
More often than not when workers are unable to log into the app it is because the effort has not been launched. In order to begin canvassing the Launch button must be selected. After setting the survey, turf, and workers, the button will be clickable.
If you are unable to log into the app double check that your log in credentials are correct. Error can happen on both the set up side and on the app side. If the the username and campaign code are not accurate the app will not let the worker log in. Usernames can be found and adjusted under the Worker tab and the campaign code is located in the upper right hand corner of the management interface.