I wanted to claim a server, and I changed its DNS from
GameTrackerClaimServer in www.playarena.ro, now I can't access gametracker it shows me this https://imgur.com/IJkHY99

what can I do?

The dns name www.playarena.ro translates to IP 89.40.105.55
There are no servers currently hosted on 89.40.105.55.

In any case you should claim your server and change the name back to the correct/normal name before making changes to configuration.

how do I not have any hosted server? It was a furious one and I reinstalled it I made it a classic, then I claimed it after I tried to change its name from the gametracker and that error appeared

You have added a server with IP 89.40.105.21

The IP address of www.playarena.ro is 89.40.105.55

If you update the dns on the server page to www.playarena.ro the server will disappear from Gametracker again.