Did you upgrading an existing on premise install of the Elastic Stack? If so, do you have the Kibana log file or console you are running it on close by to peek into it to see if maybe there is more involved error message?
That 500 could be a variety of things such as but not limited to...Networking issues, disk issues, memory issues, etc... Sometimes within the Kibana logs there is a more detailed error message or a collection of other errors that proceeds this one which would help identify what is happening
Also, do you get the same 500 error message if you try and create a custom rule or just when you try to install the pre-packaged rules?
Oh and of course, welcome to the discuss forums and thanks for taking the time to post your first post!