LevelNorm says “+0 dBTP clip warning, cannot adjust to target”. What does that mean?

It means that some peaks in your audio are so high, the average loudness can not be raised till -23 LUFS. You should manually or with the aid of a limiter lower the highest peaks and run LevelNorm again. Mark that these peaks are often very short, they usually do not have any meaning for the perception of your program so you can safely remove them.