This means that the URL in question contains at least one outgoing anchor link with a URL in an onclick attribute.
This is a type of JavaScript link - the onclick attribute defines a JavaScript action when the 'onclick' event for the link is triggered (i.e. when a user clicks the link) - and there is a URL present itself in the onclick attribute.
This means that the link destination is JavaScript dependent, which search engines are not able to follow.
Google clearly state in their documentation that onclick links are not crawlable:
The Hint will trigger for any internal URL that has at least one outgoing link, where the link includes a URL in an onclick attribute.
An anchor with an onclick attribute, with a URL defined in the onlick attribute
<a href="javascript:void(0);" onclick="window.location.href='https://example.com'">onclick link!</a>
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing.
The reason for this is that it may be intentional that certain links are not crawlable - for instance if they are only designed with user interaction in mind (e.g. 'add to basket'). Since Sitebulb cannot divine the intention of the link, it is not clear whether the link is supposed to be crawlable or not.
From the URL List, click the blue Hint Details button, which will bring up a slide-across window with specific details about the Hint for each URL.
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