The error in your first post is because you're trying to cd into something that doesn't exist. If you want to reference a file in the current directory, you should just use something like "manage.py". The syntax "/manage.py" means "go up to the very top of the directory tree, then find the file or directory there called "manage.py". Additionally, "cd" is how you navigate into directories, and "manage.py" is a file, not a directory.
The second error is because you're using the wrong version of Python to try to run the code. It looks like your website is set up to use Python 3.5, so the command to run manage.py would be this:
python3.5 manage.py createsuperuser