Permalinks Not Working

I am trying to get my permalinks set up like this. https://www.example.com/category/post_id#

So if I have a category named PC for instance I want it to be like this https://www.example.com/pc/post_id#

I went to permalinks and did custom and wrote /%category%/%post_id%

It changed the link of my post to the correct thing but it keeps giving me a 404 error. Everything on my site gives me a 404 error message. All of my other links become mislabeled. I set my .htaccess to 777 so it would write on it and it is. What am I doing wrong?