I had the same issue but I found the solution to it. Basically "origin" is another name from where your project was cloned. Now the error
fatal: remote origin already exists.
LITERALLY means origin already exists. And hence to solve this issue, our goal should be to remove it. For this purpose:
git remote rm origin
Now add it again
git remote add origin https://github.com/__enter your username here__/__your repositoryname.git__
This did fix my issue.