Unable to choose Custom Structure within permalink settings.

I’m having an issue with my permalink settings. I try to set it to a custom structure which would be https://www.mysite.com/%postname%/ and it does not work. Each time I hit save it always toggles back to the “Post Name” setting which is https://www.mysite.com/sample-post/. The odd thing is that I can set the custom structure to /%category%/%postname%/ and it would actually save. But this isn’t the custom structure I want. what is causing this issue and how can I fix it?