Warning message: In `…` : invalid factor level, NA generated

The warning message appeared because, in R, character values are by default treated as factors while a data frame is created. Thus, while creating your data frame, the variable “Type” was made a factor, and “lunch” was not a predefined level in that factor.i.e.,

fixed <- data.frame("Type" = character(3), "Amount" = numeric(3))
> str(fixed)
'data.frame': 3 obs. of  2 variables:
 $ Type  : Factor w/ 1 level "": 1 1 1
 $ Amount: num  0 0 0

To avoid this warning, you can include the argument stringsAsFactors = FALSE to force R not to convert ‘Type’ to a factor while creating the data frame.i.e.,

fixed <- data.frame("Type" = character(3), "Amount" = numeric(3),stringsAsFactors=FALSE)
fixed[1, ] <- c("lunch", 100)
 str(fixed)
'data.frame': 3 obs. of  2 variables:
 $ Type  : chr  "lunch" "" ""
 $ Amount: chr  "100" "0" "0"