Here is a solution with findInterval.

cut_points <- c(0, 12, 18, 21, Inf)
action <- c(
  'Go directly to jail, do not pass go',
  'Cast haste on the nearest archer',
  'Summon a dragon',
  'reroll'
)

set.seed(2024)
d20roll <- sample(20L, 1L); d20roll
#> [1] 2
i <- findInterval(d20roll, cut_points)
action[i]
#> [1] "Go directly to jail, do not pass go"

^{Created on 2024-09-08 with reprex v2.1.0}

Note that findInterval is vectorized so if you want to run it R times, just change the call to sample to sampling with replacement.

R <- 100L
d20roll <- sample(20L, R, replace = TRUE)
i <- findInterval(d20roll, cut_points)
action[i] |> table()
#> 
#>    Cast haste on the nearest archer Go directly to jail, do not pass go 
#>                                  35                                  50 
#>                     Summon a dragon 
#>                                  15

^{Created on 2024-09-08 with reprex v2.1.0}