The message "Unable to perform non fast forward merge on target. Please pull or fetch and merge manually" usually means that your origin branch is not up to date; or in other words, someone else has submitted to the remote repository since you created your repository and you are not up to date. You need to push the fetch button to update your origin branch and then merge the origin branch into your local branch.
For instance if you are on the master branch, push fetch, open the branch manager, select origin/master and select merge. Then you need to resolve any conflicts and perform a commit. You might not have any conflicts, but if you do they need resolving.
Once you have done this you will be able to do a push.
