To successfully scrape data from Twitter, you will need to utilize Rselenium and select the appropriate CSS. Here is a snippet of code that should help you achieve this:
library(RSelenium)
library(rvest)
rmDr <- rsDriver(browser = "chrome")
myclient <- rmDr$client
video_tweet = "https://twitter.com/estrellagalicia/status/993432910584659968"
myclient$navigate(video_tweet)
mypagesource <- myclient$getPageSource()
read_html(mypagesource[[1]]) %>%
html_nodes("#permalink-overlay-dialog > div.PermalinkOverlay-content > div > div > div.permalink.light-inline-actions.stream-uncapped.has-replies.original-permalink-page > div.permalink-inner.permalink-tweet-container > div > div.js-tweet-details-fixer.tweet-details-fixer > div.card2.js-media-container.has-autoplayable-media > div.PlayableMedia.LiveBroadcastCard-playerContainer.LiveBroadcastCard--supportsLandscapePresentation.watched.playable-media-loaded > div > div > div > div:nth-child(2) > div.rn-1oszu61.rn-1efd50x.rn-14skgim.rn-rull8r.rn-mm0ijv.rn-13yce4e.rn-fnigne.rn-ndvcnb.rn-gxnn5r.rn-1nlw0im.rn-deolkf.rn-6koalj.rn-1pxmb3b.rn-7vfszb.rn-eqz5dr.rn-1r74h94.rn-1mnahxq.rn-61z16t.rn-p1pxzi.rn-11wrixw.rn-ifefl9.rn-bcqeeo.rn-wk8lta.rn-9aemit.rn-1mdbw0j.rn-gy4na3.rn-u8s1d.rn-1lgpqti > span > div > div.rn-1oszu61.rn-1efd50x.rn-14skgim.rn-rull8r.rn-mm0ijv.rn-13yce4e.rn-fnigne.rn-ndvcnb.rn-gxnn5r.rn-deolkf.rn-6koalj.rn-1pxmb3b.rn-7vfszb.rn-eqz5dr.rn-1mnahxq.rn-61z16t.rn-p1pxzi.rn-11wrixw.rn-ifefl9.rn-bcqeeo.rn-wk8lta.rn-9aemit.rn-1mdbw0j.rn-gy4na3.rn-bnwqim.rn-1lgpqti > div > div > span > span") %>% as.character()