I don’t understand something. If the “backlink is not found”, doesn’t this mean that the link in the site has been removed. Then how can it be listed in the table on the dashboard?
If you have added the links from the backlink checker, the checker is updated slower (more than a month), so some links can be gone by the time they are shown.
That is why, when adding them to your dashboard, you will see real time results (they are all checked once they are added) and are re-checked every few days.
For the links that are archives or pages (example: page4), those links change and you should find the article/post where your link is and update the URL.