The first thing to do is configure Permalink
You Have to go into wordpress admin and settings permalink "Custom Structure"
Select Custom Structure ClassiPress enter the code standard.
Permalink default:/%postname%/
Note: The correct and Does Not Have the space at the Beginning and end: "/%postname%/"
If everything is correct, just save Permalink again and see if it works.
Check the following questions:
Permalink used is the standard
Even if that is correct, save it again.
Permalink:
See if the file ".htaccess" file was created correctly