@kmmukesh That is not a reproducible example because we cannot recreate the Subject_Visits object from the code you have supplied. Please read the guide I linked to more carefully as that will increase your chances of getting help from the community.
Since I don't have a sample of your data, I'll make up some of my own to illustrate @phiggins' solution.
library(dplyr, warn.conflicts = FALSE)
library(lubridate, warn.conflicts = FALSE)
df <- tribble(~ subject, ~ date,
"A", "2017-02-13 UTC",
"A", "2017-03-13 UTC",
"A", "2017-04-10 UTC",
"A", "2017-05-10 UTC",
"B", "2017-06-08 UTC",
"B", "2017-07-06 UTC",
"B", "2017-08-07 UTC",
"B", "2017-09-06 UTC",
"B", "2017-09-25 UTC")
head(df)
#> # A tibble: 6 x 2
#> subject date
#> <chr> <chr>
#> 1 A 2017-02-13 UTC
#> 2 A 2017-03-13 UTC
#> 3 A 2017-04-10 UTC
#> 4 A 2017-05-10 UTC
#> 5 B 2017-06-08 UTC
#> 6 B 2017-07-06 UTC
df %>%
mutate(date = ymd(date)) %>% # coerce column from character to date
group_by(subject) %>%
summarize(max_date = max(date))
#> # A tibble: 2 x 2
#> subject max_date
#> <chr> <date>
#> 1 A 2017-05-10
#> 2 B 2017-09-25
Created on 2020-03-25 by the reprex package (v0.3.0)