fatal: Not a gitflow-enabled repo yet. Please run "git flow init" first.

fatal: Not a gitflow-enabled repo yet. Please run "git flow init" first.

Please follow below steps to remove this issue:

1. Open the .git\config file
2. Remove all the [gitflow * entries and save the file
3. Close and re-open SourceTree
4. Go in the main menu, go to Repository > Git Flow > Initialise Repository (should be enabled now)