Diagnosis
You want to delete the indicator "Interval values allowed", so thatinterval values can no longer be assigned to the characteristic.
The characteristic has already been used. The system check of valuesalready assigned has found at least one case where an interval valuewas assigned to the characteristic. This contradicts what you want todo with the indicator.

Procedure
Undo your change.
Before you delete the indicator again, first check all values assignedto the characteristic and delete the intervals if appropriate.
To see a list of all the places where the characteristic is used,choose Environment -> Where-used list from the characteristicsmenu.