From R 3.0.0 onwards mean(<data.frame>) is defunct (and passing a data.frame to mean will give the error you state)
A data frame is a list of variables of the same number of rows with unique row names, given class “data.frame”.
In your case, result has two variables (if your description is correct) . You could obtain the column means by using any of the following
lapply(results, mean, na.rm = TRUE)
sapply(results, mean, na.rm = TRUE)
colMeans(results, na.rm = TRUE)
