Help on warning message "no font could be found for family 'Arial'"

General
4

When I run that code I don't have any errors or warnings (see reprex below).

The message;

is telling you that if you run warnings() in your console, R will report what those warning messages are. Also note that warnings don't necessarily stop your code. For details; R: Warning Messages

plot(cars, 
     sub="Test", 
     xlab = "Speed (mph)", 
     ylab = "Stopping distance (ft)", 
     las = 1, xlim = c(0, 25))

Created on 2019-05-29 by the reprex package (v0.2.1)

Also note that this isn't a reprex. Here's a nice video that walks you through how to get that.
Reproducible examples are useful (if not essential) to diagnosing problems like this.