What does this mean?
Aria-labelledby is being used on an element. The element this attribute is pointing to is not present on the page.
In the example below this issue would be raised if there was no element on the page with an id of dialog-header:
<dialog aria-labelledby="dialog-header">
<h2>You will be signed out soon</h2>
...
Impact on users
A missing target for aria-labelledby will likely mean that the element with the aria attribute will be missing an accessible name.
As aria-labelledby can have more than one element referenced it could be that the element with the aria attribute still has an accessible name, but it might be incomplete or misleading. You will need to determine the potential impact for a user.
For example:
<h2 id="uni-01-heading">Univeristy of Bristol</h2>
<a href="" id="uni-01-del" aria-labelledby="uni-01-del uni-01-title">Delete</a>
In the example above the delete link is meant to reference both itself (to generate "Delete") and the h2 (to generate "Univeristy of Bristol") to generate a combination accessible name of "Delete University of Bristol".
However because the h2 has a mis-named id attribute ("uni-01-heading" instead of "uni-01-title") that part of the aria-labelledby reference fails. In this instance the accessible name of the link is just "Delete".
How to fix
Check the correct id has been added to the aria-labelledby attribute.
Check the same id is present on the correct element on the page.
When working with id attributes you should also check that there is only one element with that id on the page as the browser will pick the first one it finds.