startdate <- "2017-11-05"
enddate <- "2018-01-20"

    Symbols <- c("XOM","MSFT","JNJ", "BRK.B","GE","CVX","WFC","PG","JPM","VZ","PFE","T","IBM","MRK","BAC","DIS","ORCL","PM","INTC","SLB")



    dailyreturns = lapply(Symbols, function(sym) {
      dailyReturn(na.omit(getSymbols(sym, from= startdate, to = enddate, auto.assign=FALSE, src = "yahoo")))
    })

Warning: BRK.B download failed; trying again.

Warning: BRK.B download failed; trying again.
Error: BRK.B download failed after two attempts. Error message:
HTTP error 404.
Called from: getSymbols.yahoo(Symbols = "BRK.B", env = <environment>, verbose = FALSE,
    warnings = TRUE, auto.assign = FALSE, from = "2017-11-05",
    to = "2018-01-20")