It appears that “mdbootstrap” is built in such a way that it does not support jQuery in No Conflict mode, which is how WordPress runs it. According to this forum thread they were going to add support 2 years ago, but apparently they never did.
If you must use this library, then I think the simplest solution without modifying the library directly is to add this line next to your code that enqueues the script:
wp_add_inline_script( 'jquery-core', 'window.$ = jQuery;' );
That will put WordPress’ jQuery into the $ variable so that mdbootstrap can use it. Just be aware that this could cause issues if you run any other scripts that might use the $ symbol as a global variable.
That should solve the specific error you’re reporting here (“$ is not a function”), but I can’t guarantee that mdbootstrap works with the version of jQuery bundled with WordPress. If it doesn’t, then your only option is to load a different version, but then you have to deal with the possibility that it could cause issues with other plugins expecting the bundled version.