that’s not an error, it’s a warning, and those messages should never be visible on public websites. A hacker can provoke such kind of warnings and messages and find weak points on your website, and hack it. Such pieces of information are only for developers, normally your hosting provider don’t should show such things by default.
I’m not sure if you have access to your php.ini file, there you can change the settings. These are the settings for your php.ini file:
error_reporting = off
display_errors = off
log_errors = off
If you cannot edit this file you need to contact your hosting provider.