How do I tell Google to ignore a URL that is not a page or post?

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Google Search Console is recording DAILY mobile usability errors on a URL that is the theme’s image directory.

How to indicate to no index no follow? the directory page because the exclude tool is specifically says for pages and post id’s. I tried to put the URL in that box but after saving, it just disappeared so I don’t know if it is excluded or not.

Thanks.