Как да реша този проблем?

Не би трябвало да получаваш такава грешка. Вероятно проблем в темата.

Ето едно решение, което видях в интернернет. С две вероятно PHP версията има проблем или липсващ превод.

The warning you are seeing may be being caused by text missing in a translation file, or it may be being caused by your PHP version being strict with how the code is being used within Storefront. There is a change in this section in the next release that may fix this, but you may also want to check your translation file to make sure it is correctly translating “Posted on” as shown here:

https://github.com/woocommerce/storefront/blob/version/2.4.2/inc/storefront-template-functions.php#L440

For now, if you would like to hide the warning from appearing on your pages, you can turn on WordPress debugging and logging and the warnings will no longer display, they will go to a log. https://codex.wordpress.org/Debugging_in_WordPress#Example_wp-config.php_for_Debugging