It's just warning you that the factor jnzflike contains an NA level. Whether this is a problem or not depends on what you are trying to do.
Some R functions don't report NA values (see example below) which could be misleading. So, a way to guard against this is to turn the NA values into their own level (i.e. make it explicit). You can use forcats::fct_explicit_na() for this as shown below.
table(iris$Species)
#>
#> setosa versicolor virginica
#> 50 50 50
iris[1, "Species"] <- NA
table(iris$Species)
#>
#> setosa versicolor virginica
#> 49 50 50
iris$Species <- forcats::fct_explicit_na(iris$Species)
table(iris$Species)
#>
#> setosa versicolor virginica (Missing)
#> 49 50 50 1
Created on 2020-06-06 by the reprex package (v0.3.0)