Problems creating search-engine friendly permalink

Hi,
I’m trying to convert my permalinks from the default structure:
https://www.foo.com/index.php?p=4
to something more search-engine friendly:
https://www.foo.com/post-title
So, I go to Options, then to Permalinks, and then put in the following value:
/%postname%
and then click “Update Permalink Structure”
which then creates data for me to put in my .htaccess file, which I update.
So I’ve done all of that, but nothing works: when I click on the title of a post, it tried to take me to https://www.foo.com/post-title, but brings to an error page that says that the file doesn’t exist.
What am I doing wrong?