Permalink Error

Hi

I just Start a new blog and when i change the Permalink Form the settings Default: https://localhost/ecover/?p=123 to Custom Structure: /%postname%/ and when i Click in the Categories The Category shows error

Object not found!
The requested URL was not found on this server. The link on the referring page seems to be wrong or outdated. Please inform the author of that page about the error.

If you think this is a server error, please contact the webmaster.
Error 404

But when i make it Default: https://localhost/ecover/?p=123 again then the category is working!!

What is the problem? plz suggest. I just want to make the URL SEO friendly.

Thanks.