The message saying the ad's destination URL is down or unavailable comes from a check by Google itself. Before accepting the edit, Google tests the ad's landing page, and if the page doesn't respond, it returns a policy error (destination not functional). Check that the site is up, that the final URL has no typos, and that the page doesn't block bots or redirect to a broken address. After fixing the site, try saving again. The same kind of block can happen when you duplicate campaigns whose ads point to pages that are down. If the site is working normally and the error persists, wait a few minutes (it may be temporary instability) and try again.