It is unclear from your question what you want to do if any element in the relevant column violates the 10-character constraint, but here is a simple example that might help get you started:
library(tidyverse)
digits <- tibble(digits = c(1234567891, 123456789))
digits %>%
mutate(digits = digits,
length = nchar(digits),
len_check = case_when(length == 10 ~ TRUE,
TRUE ~ FALSE))
#> # A tibble: 2 × 3
#> digits length len_check
#> <dbl> <int> <lgl>
#> 1 1234567891 10 TRUE
#> 2 123456789 9 FALSE
Created on 2021-10-05 by the reprex package (v2.0.1)