Changing attribute in R not possible because of levels -

I am trying to rename a feature in my local dataset. This is considered an easy change, but for some reason it gives me an error message stating 'Illegal factor level' when I select the attribute then this message gives the correct name other than 415 levels. It confuses me what it means but I think I can change this feature name in an easy way. Help is greatly appreciated!

Moon_Net $ GM_NAAM [406]

[1] Pseudostest-free ¢ N 415 Level: 'S-Greyhaven's Hertzbosch AA Hann Alberg Warning Message: [& lt; - In

.factor ( * tmp * , 406, value = c

  mun_neth $ GM_NAAM <- as.factor (replace (as.character (mun_neth