'git init -b <branch name>' command in terminal is throwing an 'unknown switch' error

git 2.24 doesn't have option -b/--initial-branch. It was added in git 2.28. You need to upgrade to use the option.

Or, as @matt said, create a repo and then rename the branch:

git init repo
cd repo
git branch -m master slave

prior to git v2.28

git init                              # ①
git symbolic-ref HEAD refs/heads/main # ②

① After git init, the branch master does not actually exist. Branches get created only when they have at least one commit.

② This updates .git/HEAD to contain ref: refs/heads/main instead of ref: refs/heads/master. Alternatively, git checkout -b main.

git v2.28+

As @phd said, the -b/--initial-branch option was added in git v2.28. git 2.28 also introduces a config option to specify your preferred default branch:

git config --global init.defaultBranch main

Learn more about the new init.defaultBranch setting in GitHub's blog post.

The -b flag is only available in version 2.28 or later, you need to upgrade your Git.

On debian-based Linux systems such as Ubuntu, do the following:

sudo add-apt-repository -y ppa:git-core/ppa
sudo apt update
sudo apt install git -y