I think this has more to do with handling git commands than with R or RStudio. Git thinks that changes have been pushed to the repository that do not exist in your local copy. Does anyone else have access to the repository?
I am not skilled enough with git to offer a solution with confidence. I know what I would try but I might just lead you into a disaster. You might want to ask on a Git forum where people deal with these sorts of problems regularly. You may well have to do some command line interaction with Git. In any case, I would manually copy the file to another directory to have a back up. That is probably blasphemy to some Git users, but I prefer to be safe.
The error message suggests looking at the "Note about fast forwards" in git push --help. Here is the text of that, which may help understand what git is saying
NOTE ABOUT FAST-FORWARDS
When an update changes a branch (or more in general, a ref) that used to point at commit A to point at another commit B, it is called a fast-forward update if and only if B is a descendant of A.
In a fast-forward update from A to B, the set of commits that the original commit A built on top of is a subset of the commits the new commit B builds on top of. Hence, it does not lose any history.
In contrast, a non-fast-forward update will lose history. For example, suppose you and somebody else started at the same commit X, and you built a history leading to commit B while the other person built a history leading to
commit A. The history looks like this:
Further suppose that the other person already pushed changes leading to A back to the original repository from which you two obtained the original commit X.
The push done by the other person updated the branch that used to point at commit X to point at commit A. It is a fast-forward.
But if you try to push, you will attempt to update the branch (that now points at A) with commit B. This does not fast-forward. If you did so, the changes introduced by commit A will be lost, because everybody will now start
building on top of B.
The command by default does not allow an update that is not a fast-forward to prevent such loss of history.
If you do not want to lose your work (history from X to B) or the work by the other person (history from X to A), you would need to first fetch the history from the repository, create a history that contains changes done by
both parties, and push the result back.
You can perform "git pull", resolve potential conflicts, and "git push" the result. A "git pull" will create a merge commit C between commits A and B.
Updating A with the resulting merge commit will fast-forward and your push will be accepted.
Alternatively, you can rebase your change between X and B on top of A, with "git pull --rebase", and push the result back. The rebase will create a new commit D that builds the change between X and B on top of A.
Again, updating A with this commit will fast-forward and your push will be accepted.
There is another common situation where you may encounter non-fast-forward rejection when you try to push, and it is possible even when you are pushing into a repository nobody else pushes into. After you push commit A
yourself (in the first picture in this section), replace it with "git commit --amend" to produce commit B, and you try to push it out, because forgot that you have pushed A out already. In such a case, and only if you are
certain that nobody in the meantime fetched your earlier commit A (and started building on top of it), you can run "git push --force" to overwrite it. In other words, "git push --force" is a method reserved for a case where
you do mean to lose history.