Im deathy, 3d artist, and I wanted to upload my content for first time, and i noticed that i have my nickname taken by someones elses (I have artist tag on rule34, but not here), how can i fix this? here is my twitter if needed https://x.com/DeathyKy

You mean deathy? Since you have alternate artist names, and you appear to use deathyky as your go-to name rather than deathy, you could just use deathyky instead.

As far as I know, artist names are first come first serve in the rare cases that two or more people want the exact same name.
You could add Deathy to your wiki as an alternate name, but Ruppari is right that DeathyKy is a good alternate