df is the name of the sample dataset I have created to exemplify the solution
df <- data.frame(stringsAsFactors = FALSE,
x = c(NA, "N/A", "null", "", "1", "2", "3"),
y = 1:7)
df
#> x y
#> 1 <NA> 1
#> 2 N/A 2
#> 3 null 3
#> 4 4
#> 5 1 5
#> 6 2 6
#> 7 3 7
NA stand for Not Available, and is the way of R to represent missing values, any other form is treated as a character string i.e. c("N/A", "null", "")
%>% this is called the pipe operator and concatenates commands together to make code more readable, the previous code would be equivalent to
na.omit(mutate_all(df, ~ifelse(. %in% c("N/A", "null", ""), NA, .)))