Any form that is defined as either a log form or a registration form must have at least one log field defined. These are the fields that will be displayed in the log table within the subject records. Defining a log field is done within the form builder in the field properties by checking the "show in log" attribute as shown below in red box:


This same action must be taken on the enrollment form which has been defined for the study. 

If you open the subject manager to test CRF entry and get the error shown below, its because this has not yet been done.

General Error: There are no fields defined for this study's enrollment log. At least one field must be defined to show in the log.