The $(…).modal is not a function is usually caused because scripts are loaded in the wrong order . The browser will execute the scripts in the order it finds them. If in a script you attempt to access an element that hasn't been reached yet then you will get an error. Make sure that you don't have a script above one that it requires.
Some plugins and CSS components depend on other plugins. If you include plugins individually, make sure to check for these dependencies in the docs. Also note that all plugins depend on jQuery (this means jQuery must be included before the plugin files).
Multiple jQuery instances
Sometimes this warning may also be shown if jQuery is declared more than once in your code. The second jQuery declaration prevents bootstrap.js from working correctly. The problem is due to having jQuery instances more than one time. Take care if you are using many files with multiples instance of jQuery. Just leave one instance of jQuery and your code will work.