Totally lost ranking for a targetted page - and dont understand why

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I am trying to rank this page https://www.vitari.no/regnskapssystem/visma-net/ for keyword visma.net on google.no.

At first it went really well, and I was on first page, ranking as number 8. Then it fell to second page. But then it totally dissapeared. If I go to google.no now and search for visma.net, its not on any of the pages, I have looked through them all. I have used moz and other seo tools to analyze, and try to understand what has happened. But I simply cant understand it. And I cant get rankings back. I've been fighting with this for a long time.

However - other pages on the site is ranking for the same keyword.

If anyone has an answer leading to me solving this, they will be my hero of the day!