Adding a column with an if else statement

I'm trying to cut numbers into categories to create a new column. Basically, trying to create a letter grade ("A", "B", "C", "D", "F") from scores.

I have reproduced a similar data frame to the one I'm having trouble with in the following code.

df <- tibble(score = rnorm(20:100, n = 150))

The code I wrote to add the grade column looks like this:

df_with_grade <- df %>% 
  mutate(Grade = if (score >= 90) {
    "A"
  } else if (score >= 80){
    "B"
  } else if (score >= 70){
    "C"
  } else if (score >= 60){
    "D"
  } else {
    "F"
  }
  )

The code executes with a warning:

Warning messages:
1: In if (score >= 90) { :
  the condition has length > 1 and only the first element will be used
2: In if (score >= 80) { :
  the condition has length > 1 and only the first element will be used
3: In if (score >= 70) { :
  the condition has length > 1 and only the first element will be used
4: In if (score >= 60) { :
  the condition has length > 1 and only the first element will be used

The result is, all scores are assigned an "F"