I want to clean my data in one go, but the following code doesn't work in R

I want to clean my data in one go, but the following code doesn't work. Data has factor features and numeric features as well.

findnullna <- function(x){
  for(i in 1:length(x)){
    if(x[i]=="" | x[i]=="NULL" | x[i]=="#N/A" | x[i]=="#NAME?"){
      x[i]<- NA
    }
  }
  return(x)
}


df <- sapply(df,findnullna)

I get an error

Error in if (x[i] == "" | x[i] == "NULL" | x[i] == "#N/A" | x[i] == "#NAME?") { : missing value where TRUE/FALSE needed

also sometimes I get

Error in x[i] : object of type 'symbol' is not subsettable