You need to switch off displaying of PHP errors. There are 3 steps to do this for OpenCart 3.0.
1. In the admin goto System -> Settings -> Edit -> Server (tab) and and change Display Errors to No. Then save.
2. To switch off the framework errors. Change $_['error_display'] to false in system\config\default.php. Or add
to system\config\catalog.php and system\config\admin.php
3. Next you need to set the PHP ini setting display_errors to Off. Who you do this will depend on your host. It could be adding
to your php.ini file in the OpenCart and admin folders. Or you may need to add
to your .htaccess file in the OpenCart folder. Or you may need to do something else depending on your hosting.
You will also want to check the location of the PHP error log and make sure it is set to log to somewhere that is publicly accessible. The error_log and/or log_errors PHP ini setting are what you need to change for this.