When you run git commit, Git will read from the configure variable commit.template to find a commit message template and load it into your editor for you to write your commit message.
Regarding your error, it seems that Git is trying to find the template file /Users/<username>/.stCommitMsg – but it does not exist.
Check your configuration with:
git config --get commit.template
Double-check that your username is correct. If the file does not exist, you can create it with:
touch /Users/<username>/.stCommitMsg
Alternatively, you can edit ~/.gitconfig and delete the commit.template item:
[commit]
template = /Users/<username>/.stCommitMsg