[Решено] Problem with translation post categories

Background of the issue:
I am trying to change the URL of the slug in Greek (default language) on my site скрытая ссылка. I am also experiencing a 404 error in the second language.

Symptoms:
I can't change the URL of the slug in Greek, and the second language displays a 404 error.

Questions:
Why can't I change the URL of the slug in Greek (default language)?
Why does the second language display a 404 error?