Diagnosis
You want to delete the "multiple values" indicator, so that only onevalue can be assigned to the characteristic.
The characteristic is already in use. The system check of the valuesalready assigned has found at least one case where the characteristichas more than one value assigned to it. This means that deleting theindicator would lead to inconsistencies.

Procedure
Reselect the field.
Before deleting the indicator again, first check all the valuesassigned to the characteristic and delete the multiple values ifappropriate.
By choosing function Environment -> Where-used list from thecharacteristics menu, you can find out where the characteristic hasbeen used.