Typically a "link" is referencing the '<a href=""></a>' tag.
The error code is telling you that the href="" is not pointing to, or can't be found in, the correct location.
Here is a reference to look up the particulars.
In general you want it to look like:
<a href="
#mylink">Go to my linked location.</a>
<p id="mylink">This is my linked location.</p>
- or, if it is on a different document:
<a href="htmlpage.html#mylink">Go to my linked location on a different document.</a>
It is important to note that the href and id must have the
identical name...and it is case sensitive. So ISBN and isbn are NOT the same.