Find the red error message indicator on the right-hand side:
Click it to open the Console, then click the file referenced in the error notice:
This takes you directly to the code in the file that is related to the error. Look for the red underline and copy part of that code:
Open a new browser tab and load the un-cached version of the page. You do that by adding ?nocache to the URL—this will load an un-cached, un-minified version of the page. It is necessary in order to find the original URLs of the files.
An un-cached URL would look something like: https://example.com/?nocache or: https://example.com/some-page/?nocache
Open Developer Tools, then click the 3 dots on the right side and select Search All Files:
Paste the problematic code in the search box and it will locate the file in which that code is found:
It’s not possible, unfortunately, to copy the filename from this screen. So to do that, click on the Network tab (reload the page if necessary to pull in the files). Then search for the file in the list:
Once you’ve located the file, right-click on the filename, select Copy, then Copy Link Address.