Board Thread:Support Requests - Getting Started/@comment-51.15.109.139-20200515013550/@comment-9605025-20200515014253

You may be getting that message because someone has a similar name. In order to prevent impersonation, Fandom also blocks names that are similar to existing names. - Edit:

Correction. There is a user by the name "Darkthought the nightwing". It is literally the same name you are trying to use except with a lower case "N". So I am now 99.9% certain you are getting that message due to Fandom's anti-impersonation feature.