I experienced similar results. In my haste, I pasted the full GitHub .git address of the repo before I saw that the input on the prompt said, “user/repo”. .
I tried pasting the repo address, the https clone address, the ssh clone address…
It was only when I completely reloaded the page in another browser (and slowed my roll), that I saw that the prompt loaded by saying “user/repo”.
#LessonLearned, just type in your GitHub username/repoName.