GeoForm is a configurable template that allows users to enter data through a form instead of a map's pop-up. In some instances, a field in the form is grayed out and cannot be unchecked. The field displays in the form as 'required'.
This issue is caused by symbolizing a feature class based on a unique value from a field in ArcMap. This field cannot be unchecked in the GeoForm web application when the feature class is published as a service to ArcGIS Online.
Publish the feature class without the unique value symbology from ArcMap. Set the unique value symbology for the feature class in the ArcGIS Online web map.