Diagnosis
The matrix list displayed contains entries for all availablecombinations of characteristic values and planning dates for which ausage probability is defined or can be defined. There are alsocombinations for which no usage probability can be defined, because:
* the characteristic value is not valid on this date
* the date is not a supporting point
You tried to make a new entry in the matrix list. However, you cannotenter the combination of characteristic value and date that you chose,for one of the reasons above.

Procedure
Either choose another combination of characteristic value and date,that is valid, or delete your entry.