I had this problem with a newly created git repository without any commits, by chosing "VCS | Enable Version Control Integration" in IntelliJ as described here:

https://www.jetbrains.com/help/idea/set-up-a-git-repository.html#put-existing-project-under-Git

After adding the first commit (I did this in git bash, by "git add .gitignore" and "git commit"), the IntelliJ option "+ New Branch" became available.