You are getting an error because you are trying to subset a column named New and you don't have it defined in your dataframe, look at this example, your code works if it's applied to columns that you have already defined.
z <- data.frame(stringsAsFactors = FALSE,
nonLT = c(81L, 31L, 606L),
ser = c(46L, 19L, 325L),
none = c(5253L, 804L, 8110L),
total = c(5380L, 854L, 9041L),
TRT = c("GB", "SG", "BP"))
z$nonLT[z$TRT == "GB"]
#> [1] 81
z[z$TRT == "GB",]
#> nonLT ser none total TRT
#> 1 81 46 5253 5380 GB
Created on 2019-04-21 by the reprex package (v0.2.1.9000)