How to return a error message in R?

Simply include stop() inside your function/script

If you'd like an error message, include it inside stop() like so

stop("This is an error message")

Since you don't specify what you really want, all I just can say is take a look at

?message # prints a message but not stop execution
?warning # prints a warning message but not stop execution
?stop # stops execution of the current expression and executes an error action.