Not able to start feature branch by selecting commit id option in SourceTree

Open terminal

git branch release/yourReleaseName 43331988a3b86cc7ece4d323e8ec578c65437e8847

git flow finish yourReleaseName

If you're looking for the long string, you can get it in various ways, one is to open git flow, start release, check from specific commit, select your commit, and copy and paste to notepad, and then if you cant copy and paste it to terminal, you can manually enter it (i had to do it this way)

Good luck (thank Roger for the answer)

I'm creating this answer here in case this comes up the next time I run into this. This identical question was asked on Atlassian's site too. The answer there was:

Hello! This appears to be an issue with git-flow where it will not allow commit SHAs to be used when starting a new feature (I'd suggest that you file a bug here. If you'd like to start a new feature from a branch, then I'd recommend just typing the name of the branch into the "Specified commit" box.

(emphasis mine) which worked for me.

This is a bug in SourceTree application and atlassian support team accepted it and disabled these options for hotfix branch in latest release but for other branches. Hopefully they will fix or disable these option for feature and release also, so no one else gets confused.

Bug Link: https://community.atlassian.com/t5/Sourcetree-questions/Not-able-to-start-feature-branch-by-selecting-commit-id-option/qaq-p/674112

Instead of using Gitflow, you can right click on the branch from which you need the release or feature branch to be created and select branch. If it is feature then feature/[feature name] or if it is release then release/[release name]