Subdomain Permalinks issue

I set up a subdomain as I work on a site for a client. The problem is if I want to set my permalink to /%category%/%postname%/, but when I do I get a server 500 error. I am guessing it has something to do with the web-config file, but I can’t figure it out. Any help would be appreciated. My subdomain url I want will be https://www.sub.main.com