It looks like you are using a curly quote, “, in the command and you need to use a plain double quote, ".
Here is what you write
Bank.Bill.Data <- read.csv(“C:/Users/bulli/Downloads/90 Days Bank Bill Data.csv”, header = TRUE, as.is = T)
and it should be
Bank.Bill.Data <- read.csv("C:/Users/bulli/Downloads/90 Days Bank Bill Data.csv", header = TRUE, as.is = T)
You can also use single quotes for the file name, though they too must be plain straight quotes.