I just faced this problem and resolved!! Usually it happends when you rename a github repository and try to create a new one with previous name.

Steps that I followed to resolve:

step - 01: Just copy the gitbut repository link that is showing below the "Push your code to github", In my case it was [https://github.com/sydurrahman21/test1] and hit the browser.

step-02: It will redirect to the existing repository page

step-03: Just delete and try again from visual studio.

It should resolved now. Thanks and Happy coading!!!