Ragic does not allow you to delete a field if it is referenced by other sheets. For example, the field may be used as a Linked Field in a Link and Load relationship or in a sheet generated from a Subtable.
This restriction is in place to prevent other sheets from referencing a field that no longer exists. If you still want to delete the field, you must first remove any Link & Load relationships associated with it. You can click the sheet link in the warning message to go directly to the related sheet.
Similarly, if a sheet contains fields that are referenced by other sheets, the system will prevent you from deleting that sheet.