If you are trying to complete a market sort via the control interface, either at the control or connecting to the control remotely, and you have Omni-Select Enabled within the Administrative settings, then you will not be able to adjust the holdbacks. When Omni-Select is enabled, it is assumed that you are using Omni-Select to manage the sorters, and thus some functionality is only available on the Omni-Select interface to ensure data is properly synced and saved between the two systems. If you want to manage the sorters on the sorter interface rather than on Omni-Select, go to the main Menu -> Administration -> Omni-Select -> Disable.