Next time please provide a more detailed description on how to reproduce the problem. A single message is often too little information for others to help you.

Anyway, it seems you are using Windows. If you are on Windows 10 (November 2019 update) or newer version, I'd suggest you change your locale to UTF-8 from the control panel.

BTW, with a quick Google search, this warning seems to be from Perl. I'm not familiar with Perl and don't know why it emits this message.

Please see rstudio/tinytex#439 (comment).