When I run "git push" I get "tip of your current branch is behind its remote" but the current branche has no upstream tracking branch

I am working on this local branch X and when I try to push using git push -u origin X the error message is :

! [rejected]        X -> X (non-fast-forward)
error: failed to push some refs to "********"
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

So I run : git pull And an error message also appear :

There is no tracking information for the current branch.
Please specify which branch you want to merge with.
See git-pull(1) for details.

    git pull <remote> <branch>

If you wish to set tracking information for this branch you can do so with:

    git branch --set-upstream-to=origin/<branch> X

Solution

You have a local branch named X, without tracking information, so there is no corresponding upstream.

Then you are trying to push it to origin as X. So far so good... but there is a branch in that remote with that name, so your push is only accepted if it is a fast-forward, which it is not.

When you do git pull you try to merge/rebase the tracking branch, but your X does not have one! so it fails.

You have basically two options:

Add the tracking information: as git helpfully prints in the console: git branch --set-upstream-to=origin/X X, then git pull, git status, git merge, git rebse, git push without arguments will default to the tracking branch, and it will work as expected.
Kindly refuse to add tracking information. Then you must do the full commands: git fetch origin, then git rebase origin/X or git merge origin/X, and git push origin X.

Since you are trying git push -u, and -u means "add tracking information", I guess that you want option 1.