Diagnosis
You want to delete the "Additional values" indicator.
The characteristic has already been used. The system check of allassigned values has found at least one case where the characteristichas been assigned a value that is not one of the allowed values.

Procedure
Undo your change.
Before you delete the indicator again, first check all values assignedto the characteristic and delete or change the values which do notconform to the allowed values.
To see a where the characteristic has been used, choose Environment-> Where-used list from the characteristics menu.