Git: Needed a single revision error

In your case, there is no HEAD~2, since you only have 2 commits, hence the "Needed a single revision" error message.
Try:

 git rebase -i --root

see more about --root at "Change first commit of project with Git?"

This doesn't apply to your case, but may help others. If on Linux, make sure HEAD is capitalized. If you use lowercase head like the first example below (because you are used to working on Windows or Mac and those allow lowercase head), you will get the fatal: Needed a single revision error!

Or you can use @ as an alias for HEAD, then you won't need to worry to forgetting to capitalize it.

# wrong on linux
git rebase --interactive head~2

# correct on linux
git rebase --interactive HEAD~2

# correct on all
git rebase --interactive @~2