What is wrong with my function and for-loop?

I am currently trying to count the absolute number of countries in a long string. I have loaded a data frame named "countries" with column "Countries", consisting of all countries in the world. I want to make a function that searches any string, loop over all the country-names in my df and return the sum of occurrences of any country-name. (I.e. the total number of countries mentioned)

Code:

number.of.countries <- function(str){
  # #Initialize
  countcountry <- 0

  # #loop over all countries:
  for (i in countries$Countries){

  # #Logical test:
    countries_mentioned <- grepl(i, str, perl = T, ignore.case = T)

  # #add to the count
    if (isTRUE(countries_mentioned)){
      countcountry <- countcountry + str_count(str, fixed(countries$Countries[i], ignore_case = TRUE))
    }     
  }                                                                            
  #Output
  return(countcountry)
}



###When running the function:
> number.of.countries(str)
[1] NA