Plugin stop the loading of the website

I have started to had problems with plugin.

It stops the site.

I have this message in the register log of the server

AH01071: Got error ‘PHP message: PHP Warning: Use of undefined constant AUTH_KEY – assumed ‘AUTH_KEY’ (this will throw an Error in a future version of PHP) in /var/www/vhosts/valenciablog.com/httpdocs/wp-content/plugins/ad-inserter/ad-inserter.php on line 10557′, referer: https://www.valenciablog.com/

Thanks

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