I can not deploy a simple website which have just HTML and CSS code in heroku. Also, I have another website where I have used HTML, CSS, Javscript and Bootstrap.
The error message is: ! [remote rejected] master -> master (pre-receive hook declined)
You may have the branch restrictions in the master branch of you git repository. You need to check the repository settings of your git and then check branch permissions. If there is no one who have write access in your repository, then add the user and try again deploying the website.