Posts Do Not Show Up Under Categories

I created a new WordPress blog https://www.exercisemydog.com (GoDaddy the host on Linux server). If I click on Categories in the sidebar, I do not get a list of posts for that category. In fact, I get a 404 message.

The categories do show up on admin page and I can assign a category to the posts.

I’m using a theme called Ultraclean but I have the same problem no matter what theme I use, even the WordPress default. There is no category.php file.

I’ve have searched support but do not find a similiar problem. I ‘d appreciate a hint as to where I would start to fix this problem: database, php file, etc.