The issue notification remarks that a particular URL contains at least one outgoing anchor link with the URL in the onclick attribute.
What triggers this issue?
An issue notification will be triggered for any internal URL that contains an outgoing link with the URL in the onclick attribute.
How to check the issue?
It will not do any harm to your site. Nevertheless, it is considered an error and is placed in the scan report to notify the site owner.
Pages having link with a URL in onclick attribute check is good for your website but not enough to get good SERP positions!
To detect not only the issue but other kind of site level and page level problems, just make the full site audit.
Why is this important?
How to fix the issue?
This issue is simply an alert. You do not have to solve the problem immediately. SEO optimization software simply informs web admins about the error.