Fatal: Not a gitflow-enabled repo yet. Please run ‘git flow init’ first

I got it working by doing the steps mentioned by jpfl @ answers.atlassian.com:

Although this is an old post, just wanted to add to this since I’ve
gotten stuck on this same error. Was able to resolve by doing the
following:

1. Open the .git\config file OR Repository -> Repository Settings -> Remotes -> Edit Config File (Sourcetree 2.7.6)
2. Remove all the [gitflow * entries and save
   the file
3. Close and re-open SourceTree
4. In the main menu, go to
   Repository > Git Flow > Initialise Repository (should be enabled
   now)