Hello! Have you by chance recently removed an SD card that was previously installed with the downloaded software? Are you still logged into the same Nintendo Account? If yes, you should be able to redownload the game via the eShop. If you confirm that neither of these are the issue, I would reach out to Nintendo and see what they can do for you.
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I haven't played in ages, so I don't know if this will be helpful or not, but are your Switch and game up-to-date? I know in the past, I've had issues when I needed to update something. Anyway, I hope whatever it is will be a simple fix!
Upon looking elsewhere on Google, one person had a faulty SD Card, so the Switch thought the game was downloaded but couldn't run, and it threw that error. If you have another SD Card, try downloading NH to that one and see if it works, or try downloading NH to the Switch itself. It's a bit of a large download, so it'll take up a considerable amount of space, but it'll help you with troubleshooting.