Notices /warnings are nothing to worry about. it has nothing to do with the actual functionality. but if you find any fatal error only then you have to worry about it.
What are PHP Notices?
PHP notices are the least important. As stated on the official PHP website, you will see PHP notices when:
the script encountered something that could indicate an error, but could also happen in the normal course of running a script.
Refer: https://www.ostraining.com/blog/wordpress/warnings/.
NOTE: THE ABOVE LINES ARE NOT AN ISSUE. THAT IS JUST NOTICE, YOU DO NOT NEED TO WORRY ABOUT IT. EVERYTHING WORKS FINE.
you could hide these notices, follow this link: https://www.wpbeginner.com/wp-tutorials/how-to-turn-off-php-errors-in-wordpress