STP-0021: Non linear locations must be unique under scheme
This error will occur if the Activities to Schemes flag has been un-checked and the operation location is not an offset within the scheme location.
Scheme Manager User Guide: If the ‘Activity to Scheme’ flag is not set, i.e. each Scheme Location has its own set of distinct Operations, and the Scheme Location is based upon a linear group or Datum Element, the Start and End measures for each Operation are displayed relative to the selected Scheme Location. This may be amended if required.