I received the below given message on the header of my website.

Hello,

Notices are not errors, not even warning messages. They are just messages to help developers to know something they did should be checked but nothing important.

You can read more about it here: https://stackoverflow.com/questions/4624474/php-difference-between-notice-and-warning

Usually (in most WordPress installations), you have those messages but they are hidden since you only need to check them if you have some problem on the website and you need to debug WordPress.

So you can (and you should, since you don’t want to show those messages to your users) disable displaying those messages, just as this guide explains: https://www.greengeeks.com/tutorials/article/disable-wordpress-php-error-messages/

Looking forward to help you.