no I mean there are some files in the repo already. How did you push them?
Because now, I'm guessing your local repo is no up-to-date with the remote (the one on GitHub).
So to fix it, run the following (after making sure you saved the content of what is on github somewhere because it will be overwritten):
git remote add origin https://github.com/KikeSz/Method-Sketch-Plugin
git add .
git commit -m "first commit"
git push -f
skpm publish prerelease