Scripts generated by ApexSQL Manage in particular rule cannot be edited, and this is correct, the predefined (default) health check rules in ApexSQL Manage cannot be edited or deleted.
However, this rule can be used as a template base when creating new custom rules, and this is some form of modifying.
Please refer to the article below on this topic:
https://knowledgebase.apexsql.com/sql-server-instance-health-check-creating-rules-from-templates/
Additionally, once this rule is executed, in the FixSQL tab or in the FixSQL editor this script can be modified further to suit once needs.