An error message after adding product in Joomla's article.

Hi,

First, please note that this isn't an error message. It's a deprecated message intended for developers to know what to modify in their code for proper support of futur versions of PHP.
On a normal live website, you don't want to display these. Normally, the server should be configured to not display these. I would recommend checking with your hosting how to not display deprecated messages.
Also, you can turn off the "error reporting" setting in the Joomla configuration and that will hide the messages, including deprecated messages.
Also, this error message indicates that the code which needs modification for futur versions of PHP is in Joomla, so I'm not sure we can do anything about this on our end. This code might actually have already been changed on Joomla 5.