hi,
It's just a warning. You do not need to worry about this. please disable PHP debug option.
go to files -> look for wp-config.php -> open then look for WP_DEBUG and make it false.
Like this: define( 'WP_DEBUG', false );
warnings are nothing to worry about. it has nothing to do with the actual functionality. but if you are saying this i will have a look on this please share page url & credentials in a private reply.
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/.
Please simply disable the wp_debug :
https://codex.wordpress.org/WP_DEBUG
for checking the issues that you have mentioned we need your site url and admin credentials to check this out.
please share them in private reply.