By default, ArcGIS Server services apply a lock to underlying dataset schemas that does not allow the addition of fields to an geodatabase feature class if it is deployed to ArcGIS Server. This is reflected in the software if the following error is returned.
Error: Cannot add field to the selected table/feature class because it is in use by another application/user. Lock request conflicts with an established lock.
Note: Schema locking is used to avoid modifications to the source dataset while the service is running. For more information, refer to the following document, ArcGIS Help: Disabling schema locking on a map service.
To add fields to a geodatabase feature class, stop the service running in ArcGIS Server. Stopping the service abruptly, forcefully stops and prevent other applications or users from accessing the service. To stop the ArcGIS Server services, administrative credentials are required. For more information on database server administration permissions, roles, and granting server administrative permissions to other users, refer to the following document, ArcGIS Help: Alter permissions.
The ArcGIS Server service is in an idle state when stopped and users may update the source dataset schemas. If the service is not idle and users continue to update the source dataset, unexpected behavior may occur, which include queries failing, fields and/or layers missing, as well as service unavailability.
Note: Before stopping ArcGIS Server services, ensure no applications or users are in the midst of modifying the source dataset. Refer to the following document, ArcGIS Help: Disconnect users from dekstop or workgroup geodatabases for the best approach to stop an ArcGIS for Server service.