The elementor plugin conflicts with two functions of the BPS admin.php file
How can we fix this?
PHP Notice: Undefined variable: tag in /wp-content/plugins/bulletproof-security/admin/includes/admin.php on line 733
PHP Notice: Undefined variable: tag in /wp-content/plugins/bulletproof-security/admin/includes/admin.php on line 703
Those php errors are just generic nuisance php errors that you can disregard since they do not cause any issues or problems. They will eventually be taken care of. Probably in the next BPS/BPS Pro version releases. Do you have WP_DEBUG or some other debugging option (in another plugin or theme) turned On? Elementor is not causing these php errors unless of course you have an Elementor debugging option turned On.
Now that is odd that it is being logged repeatedly. You can change your PHP Error Log settings on the P-Security > ini_set Options tab page > Error Logging option > change the option setting to: E_ERROR|E_WARNING|E_PARSE and click the Save|Enable Options button. This will log Fatal run-time errors, Run-time warnings (non-fatal errors) and Compile-time parse errors, which are really the only important/critical php errors that you need to log.