This might be a bug with the Git Visualization tool. Via the command line, if your branch can already perform a fast-forward merge, Git won’t let you do a rebase. However, the visualization tool allows for it.
Suppose you create a new branch `experiment` and add 1 commit. Then you try to do a rebase on master. You’ll get the following error.
My guess why this makes sense is because Git is protecting you (or others from you) from rewriting history. In other words, there’s nothing to rewind, so no rebase.
However, the visualization tool allows for it, but I think this is a bug. Hence why always important to experiment IRL!