DigiCORA calculates a total ozone normalization factor NT according to equation E-3-17 of GAW Report 268. The factor is calculated from the total ozone ΩT and the total ozone reference value ΩC you have given as part of the ozone preparation data. If the reference value is not given, DigiCORA does not calculate the normalization factor.
As the ozone data is not scaled with the normalization factor, the value is reported as a negative value following the recommendation in Section 5.2.1 of GAW Report 268. You must interpret the normalization factor only as a quality indicator for the measured profile.