Hi Petro,
If you hover over the checkbox, there will be message explaining why it's disabled:
You cannot modify the setting for this page type because there already exist pages based on this page type. If you need to edit the value of the setting, delete all pages based on this page type in the 'Pages' application.
Basically, if there are any instances of this page type, you need to delete those first, enable URL feature and create pages back.