Using the {palmer penguins} dataset to make a minimal reproducible example, this approach seems to work:
library(tidyverse)
library(palmerpenguins)
data <- palmerpenguins::penguins %>%
mutate(sex_f = factor(sex))
ggplot(data) +
aes(x = species, y = culmen_length_mm, color = sex_f) +
geom_boxplot() +
geom_point(position = position_jitterdodge()) +
scale_color_discrete(name = "Sex", labels = c("Female", "Male", "Missing"))
#> Warning: Removed 2 rows containing non-finite values (stat_boxplot).
#> Warning: Removed 2 rows containing missing values (geom_point).

Created on 2020-07-04 by the reprex package (v0.3.0)