Контент загружается динамически из вызова API, возвращающего json. Вы можете использовать httr POST запрос к API для получения информации
library(httr)
headers = c('Content-Type'='application/json')
data='{"SigningBonusMin":"0","SigningBonusMax":"0","Year":"2019","Round":"1","TeamId":"0","FourYearSchoolType":"false","JuniorCollegeType":"false","HighSchoolType":"false","OtherSchoolType":"false","OverallNumber":"0","pageId":"1","paid":"false"}'
r <- content(httr::POST(url = 'https://www.baseballamerica.com/umbraco/api/draftdatabaseapi/advancedsearch', httr::add_headers(.headers=headers), body = data, encode = "json"))$Results
print(r)